[System] Given the following post, respond as if you're commenting on Reddit:

[User] [ELI5] How did Duck Hunt work back in the day, with a Gun pointing to the screen?

[Assistant]
The Zapper is basically a simple light detector with a lens at the end of the barrel. Basically, think of it like a really really low megapixel camera.

When you press the trigger on the Zapper, the whole screen except for the position of the ducks is drawn as black for a frame or two. The ducks show as white squares against a black background.

Since old CRT Televisions did not give off any real light in black zones, the Zapper only had to "look" to see if it was aiming right at a bright light or not. If it was dark or black, no duck. If it was bright white, duck. The flash would then revert to normal graphics and continue on. If you slow it waaaaay down, you can actually watch it happen. It's pretty neat.

This is also why you could aim the Zapper at a light bulb and pretty much never miss. It would always detect bright light and thus register a hit.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
Why do "The World's Strongest Men" all look (for want of a better term) fat?

Title pretty much sums up my question, why do they appear fat in comparison to a bodybuilder who has a defined 6pack?

[Assistant]
If a body builder were to compete in the same competitions, they would most certainly die or become seriously injured. The more muscle you put on, the more defined the layer of fat becomes. Body building is about minimizing that layer of fat, tightening the skin, displaying vascular definition, and selectively building muscle **for aesthetics.**

Strength training is about coming to a balance of power from muscles, protective layers of fat, and flexibility. It's not to look pretty. It's for utility.
Think of it this way: A sports car looks nice, but isn't all that powerful when you compare it to a farm tractor.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
ELI5: What is it that I am seeing when I have my eyes closed.

In total darkness or in the light, when I close my eyes, I still see a dark static like effect, like millions upon millions of tiny dots in darkness. What is it that I am seeing? I can't think of any other way to describe this other than to say close your eyes and tell me what exactly do you see. I assume that I am not the only one that sees this and I am not crazy.

[Assistant]
They are called Phosphenes which basically mean you see light without a light source.

This is caused due to mechanical stimulation of the retina when you either ub your closed eyes or forcefully shut them.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
CMV: There is no purpose to rating the Door Dash Delivery driver.

So I'm new to using Door Dash and it is possible I don't understand the purpose of this feature. But I noticed that you can give the driver stars for performance. However,  I don't see any purpose to rating the driver:

1. I cannot choose a prefer driver.
2. I cannot choose a driver after the order has been submitted.
3. When I'm choosing a tip, it doesn't tell me which driver it will be for. So I don't know if I should change my tip based on level of service.
4. I cannot give a tip AFTER the food has been delivered.

So overall, I seems to be a useless feature of the app. Please CMV!

[Assistant]
It's so they know which Door Dash Delivery drivers are doing their job and which ones aren't. If the drivers score goes below a certain level they will get dropped from the system.

TLDR

It's not for you, it's for the system.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [Marvel] Can Shadowcat move through Invisible Woman's force fields?

[Assistant]
From what I've seen in the Danger Room files, she can phrase through energy but it takes extra effort.

If the Invisible Woman just has one state for her shields then once that effort is made then Shadowcat can go though.

If the Invisible Woman can modulate her fields where its different then Shadowcat will have to continuously make a effort but never get though.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [DC] So is the Gotham Judicial system ever going to find a more permanent solution for Joker? This Arkham Asylum shit clearly isn't working.

[Assistant]
That's not really how judicial systems...work?

Like, you can't give someone the death penalty on the grounds you *really* want to. It's very possible the state Gotham's in doesn't have the death penalty so they literally can't "find a more permanent solution" however much they want to. Even if it does, The Joker has been found not guilty due to reasons of insanity, and obviously you can't execute someone who has been found not guilty.

The judicial system is *very consciously designed* so that judges don't have the power to have defendents shot in the street whenever they feel like it. As much as this might enable one murder clown, it's probably overall best to keep it that way.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
Why do governments aim for a 2% inflation target?

Most governments(or central banks) of developed economies aim for a 2% inflation target. Why don't they have their inflation target at 0%? I was thinking for the central bank to increase aggregate demand just enough by printing money to meet the increase in long-run aggregate supply. This leads to economic growth, but no inflation.

[Assistant] Positive inflation rate enables central bank to provide stronger stimulus during a downturn. There is a lower bound for nominal interest rates at (or slightly below) zero. It means that real interest rate (nominal interest rate minus inflation rate) cannot go negative when inflation is at zero. In contrast, when inflation rate is positive, central bank can make real interest rate negative to stimulate consumption and investment further.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[Lord of the Rings] why didn't Golum age like Bilbo did?

Once Bilbo was separated from the ring, he aged very quickly. Golum was separated from the ring much longer, why didn't it affect him the same way?

[Assistant]
Gollum had the ring for 500 years; Bilbo had it for only 60, and he got out just in time.

The Rings of Power fundamentally change the spiritual and physical natures of their bearers: just look at the freaking Nazgul. Bilbo had it for a relatively short time, as far as these things go, and his humble nature and strong friendships prevented it from really warping him until the very end. Bilbo gave up the ring of his own free will, which meant most of the ring's influence on his body and spirit went with it. That's why his body resumed aging at the normal rate for a hobbit immediately after.

Gollum had the farthest thing from a humble nature, and killed his only friend after taking one look at the ring. The Ring corrupted him completely in a very short time, and then had centuries to dominate him without interference. Then, the Ring was taken from him against his will, which only increased the Ring's power on him. By then, Gollum was not really a hobbit anymore: he was a creation of the ring, twisted and spiteful, with only the barest cobwebs of Smeagol remaining.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5 why other sects of Christianity generally do not like Catholics.

[Assistant]
At one point in time, the Catholic Church was the *only* church...in fact, the word catholic means "universal".

Every major branch of Christianity at some point in time broke away from the Catholics.  Not only did they disagree on religion, but it lead to centuries of war in Europe, with one king trying to change the religion of his country, and the next trying to change it back.  This has led to some hard feelings that endure today.

Check out the Reformation and the Counter Reformation for more info.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
would it be possible that firing unproductive workers would actually shrink the economy at a macro level?

let's say that a worker's marginal cost (their wage) is higher than their marginal revenue product, making them a net burden to the firm. in this case evidently an unproductive worker like this would be harmful to the firm and negative to the firm's production, and thus a firm would likely let go of this worker, which theoretically should be beneficial to the economy at first glace because the worker took in more than they actually produced.

however, by letting go of this worker, they would also lose their wages and would lose wealth. as such, at a macro level, having unemployed workers (even if they are unemployed for good reason) might be negative to the economy at a macro level because aggregate demand would decrease to some extent due to the loss of wealth.

so, would it be possible that laying off unproductive workers would still be bad for the economy at an aggregate level due to this shift in aggregate demand? or would this effect be negated somehow?

[Assistant]
If the worker requires a wage above their MPL, it must be because their opportunity cost of working that job is high. If they didnt have better work to be doing (or feared the consequences of a layoff), they wouldnt require such a high wage to do that job. So firing them is probably better for all parties in the long run.

In the short run, the frictions that make it hard for workers to immediately match to a new job would cause problems. (One worker probably has little impact on the macroeconomy, as opposed to massive nationwide layoffs) That said, such frictions make workers more willing to stay at their current job for a lower wage, so for a given wage level, the presence of short run costs means the long run gains are likely to be larger.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[Marvel] What does J. Jonah Jameson think of the Punisher?

How does he usually report on himpositively or negatively?

[Assistant] There's no real way to report positively on Punisher. He's the Punisher. While JJJ certainly has his prejudice against masked vigilantes and Punisher doesn't wear a mask, HE KILLS DOZENS OF PEOPLE AT A TIME.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] Is it only a coincidence that the moon's rotation matches its orbit around the Earth (which is why we only see one side)? Will that rotation change, or will someone ever be able to see the back side of the moon from Earth?

[Assistant] No it's not a coincidence, it's called tidal locking. The larger object essentially induces a drag on the rotation of the other, until its rotation synchs up with its orbit. All the large moons in the solar system are tidally locked, and Pluto and Charon are locked with each other.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[Marvel] What is the odin force?

What's the origin of the odinforce? How does it work? What powers does it give?

[Assistant] The Odin-Force doesn't have a defined origin, or an exact list of powers. It's simply the deific power that grants Odin the vast power that he has, and makes Thor essentially indestructible when he has it in his command.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [LOTR] If Gandalf had taken the Ring, would he become independent dark lord or just another Saruman that serves Sauron?

[Assistant] An independent Dark Lord. Sauron's fear was that either Gandalf or Aragorn would get the Ring. Those two above all other's had the Will to use the Ring's power to overthrow him. They would fall to the evil of the Ring, but not before defeating him first.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
If quarks cannot be separated and require an infinite amount of energy to separate , how can neutrons or protons that are made of quarks turn into electrons by certain processes?

Although I cannot remember off the top of my head , there is a processes in which a proton or neutron can gain/lose positive or negative electric charge and turn into an electron or vice-versa. How is this possible if quarks cannot be separated ? Electrons are elementary particles , so how do quarks that cannot be separated form electrons?

Thank you for taking the time to answer this question . I did google this , but results did not yield answers that sufficiently explained the process and mechanics itself.

[Assistant]
Quarks definitely don't require infinite energy to be separated. The thing you're thinking of is that you can't have a lone quark.

The reason for this is that when you try to separate two quarks the potential energy between increases (instead of decreasing like in electromagnetism and gravity). At some point the potential energy is high enough that it becomes energetically favourable to create two new quarks that quickly pair up with the ones you were trying to pull apart. You managed to separate the two original quarks, but they quickly found some friends.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[The Fifth Element] Why does the government want to increase unemployment?

Zorg's assistant says the government is worried about the economy "heating up". What is the negative here?

Also wouldn't Zorg's response of firing double the amount asked for cause problems too?

[Assistant] Judging by the quality of life and dirty conditions, I'd guess the government is pretty corrupt and self-serving. The bureaucrats probably care more about self-enrichment than the good of the citizens. There must have been some officials that would benefit by a bad economy. Not surprisingly, they deal directly with Zorg rather than via official government policy.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
ELI5: Why does grunting, groaning or generally making little noises when you are in pain help the pain level go down?

I have had pain ranging from migraines to broken bones and the only consistent thing I've noted is that a groan, moan, grunt or some other form of low vocalization seems to help the pain level go down a bit, even if it's only temporary. Why is that?

[Assistant]
your brain can only take a certain amount of input, and can only focus on so much at a time.  By vocalizing, you're focusing your brainpower on an output and a feeling other than the massive pain input that your brain wants to focus on.

Another method that's been proven to work are things like virtual reality sims where someone can explore a complex area and focus on that experience.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [Star Wars] Do Force users have to physically hold their lightsabers while dueling? Can't they just use the Force to control their lightsabers' movement hands-free?

[Assistant]
That degree of precise, continuous control takes a great deal of focus and concentration.

At which point you don't see the saber or blaster bolt coming from your opponent who *isn't* showing off like an idiot.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
Eli5: What does it mean when music is played in a certain "key"

I understand musical notes but nobody has been able to explain to me what a "key" is in music.

For example, how does "beethovens 3rd symphony in e-minor" differ from "beethovens 3rd symphony in f-minor"?

[Assistant] A key has two parts - the selection of notes you use, and the specific note you choose to be the "tonal center" or home note of your piece.  There are 12 notes in all, and you typically choose 7 of them to make up your scale.  If you think of the numbers on a clock, and you mark 7 of them as "your" notes according to a pattern\*, and one as your tonal center, you can get an idea of how that works.  The tonal center is the one you typically come back to in order to give a sense of finality to your musical phrase or passage.  Anyway, in the examples you use, E and F are the tonal centers, and the "major" or "minor" indicate the pattern by which you choose your 7 notes.  In E minor, you'd center on E, and choose your 7 notes based on a "minor" pattern\*.  In F minor, you'd center on F and choose your 7 notes based on the same "minor" pattern.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [DC] If someone found the cure to Mr Freeze and his wife's conditions, would he accept it and retire from villiany?

[Assistant]
Depends on the version. Arkhamverse Freeze would almost certainly give up his life of crime if she was cured, since he did it anyways when she thawed out too early in the Arkham Knight DLC.

In the comics, especially post-New 52, Freeze is just a nut that had never even met Nora before she was frozen. He just found her while working at Wayne Enterprises and became obsessed with curing her, so hed still commit crimes whether she was healthy or not.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
ELI5: Electrical engineers: What is the functional difference between DC and AC power?

 I understand the conceptual difference - electrons flowing one direction with DC and flowing in alternating directions for AC. But I am a coffee equipment technician and I've always wondered why equipment manufacturers choose to use DC power at all. For example some espresso machines use 24v DC solenoid coils, while others use 110v or even 220v AC coils to do the exact same work. Why? Any why are heating elements (as far as I have seen) always AC?

I'd prefer an explanation like I am five, but if there is a better sub for this question I can move it. thanks!

[Assistant] AC can be easily stepped up and down to different voltages with a transformer.  DC voltage regulation (especially boosting it up) is significantly more complicated, and until relatively recently it was impossible to do in an efficient and cheap manner.  This makes AC better for long distance transmission, as you can step it up to very high voltages, transmit it with low loss due to the lower current, and then step it back down to go into homes.  So that's why power coming out of your walls is AC.  Heating elements are usually AC simply because it works fine, and you don't have to convert anything.  DC is used in any digital electronics because those electronics communicate by varying voltages, and if your voltage is varying sinusoidally already you can't easily do that.  For your espresso machine, they may have already converted everything to DC, so it was simpler to make everything inside the machine run on DC.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: Why old Black & White films and TV seem to have better picture resolution than stuff filmed in the 80s and 90s?

[Assistant] Because they were filmed with...well...film. Actual photographic movie film has an excellent resolution even when converted to digital formats. The reason why a lot of the 80's and 90's shows have a lower quality is because they were using the relatively new video tape technology which reduced the resolution and did not have a long shelf life if improperly stored.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5:Why do car companies use dealerships as opposed to selling directly to the customer?

[Assistant]
Because "back in the day" when cars had to proprogate across the country it was not feaisable to run a nationwide network of wholy owned retail stores.

So the solution to this problem is the dealership model.  However, a drawback of the dealership model is that the manufacturer has an extremely large amount of influence over how the dealers do businesses.  So laws were brought in to protect the dealers from the manufacturers being bullies.

And so we have the mess as it stands.  The laws protect the dealers, even if the manufacturers see a better way to do businesses.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: Why is it, when you selected "remember me" on a website, but don't sign into a website for a long time, the website always forgets you?

[Assistant]
Other answers have pointed out the how:  "stored logins use cookies and cookies have expiration dates" but not the why.

Cookies need to expire after a relative short amount of time or they would be too dangerous.  If they didn't expire, every old computer that had a drive in it with a bunch of browser cookies is a ticking time bomb for thieves and scammers.  If your laptop breaks and you throw it out, it doesn't take much skill for someone who gets their hand on it to extract the data from the drive, and if that drive let them log into Amazon, or your doctor, or your bank account, that would be terrible.

So cookies all have expiration dates on them so the damage from a thrown out computer is limited to a specific amount of time.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: How are fireworks made to explode in complicated shapes and patterns?

[Assistant]
The payloads for "sky" fireworks (not the part that delivers them to the sky where they explode) are made up of a number of different parts. A quick summary for some of the main ones:

- Fuses - do not explode, but burn at a set pace. When a part of a firework is not yet visible, it's burning a fuse (or other time-delay part) so the visible components will be delayed..

- Explosives -A small charge that blasts other parts of the firework in different directions and in different patterns. Other parts of fireworks are packed around the explosive so when it detonates, it forces the other parts out in various patterns. If you pack everything around the explosive in a single circle, you get a flat disk firework display. If you pack everything around it in a sphere, you get a round firework display.

- Black powder - the bang, or "report". Not visible, just loud when it goes off. Other powders can produce smoke, which is important to daytime fireworks.

- Stars (or other visible elements) - Often pellets, these burn brightly once ignited. Also can include 'sprays' that fountains make.

- Propellant - Sometimes parts of fireworks have extra little 'rockets' on them that create thrust so they zip off in different (and sometimes crazy) directions.

- Cardboard/paper - makes containers to hold the parts together while keeping the pieces separated. There can be a lot of it in larger, more complex fireworks.

- Chemicals - different salts and other chemicals burn with different colours. A star with copper salts added will burn green, just like some of those crazy salts you can buy to turn your campfire different colours.

So a standard "Peony" firework (very common in commercial displays) is a sphere of short-duration Stars with an Explosive in the middle which blasts the stars out in a sphere pattern as they ignite and burn. A very large one can actually force smaller explosives-surrounded-by-stars out - the first explosion sets off the fuse for the second explosion as it flies away so it'll detonate a bit later.

Finally the colours are created by different chemicals added to the stars. Take a long star, add one chemical to one end and a different one to the other, and it'll change colour as it burns.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5:Why can't you smile in your passport photo?

[Assistant] Go to a mirror and smile normally. Now smile showing teeth. Now smile absurdly wide. Now smirk. Facial expressions can seriously change the general shape and outline of your features. The goal of a passport photo is to be as easily identified as possible... so removing all variables of expression gives you a clear baseline of what the person's face looks like.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: Why is it that I can go all day (8+ hours) without my skin getting oily but if I fall asleep, even just an hour nap, it is almost instantly grease city?

[Assistant] Probably because rubbing your face against the pillow is going to get it hot and sweaty, and there will be a build up of dirt on your pillow from previous nights. If you have wax in your hair, you may be rubbing that into your skin too when you lie down.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [Star Wars] Does fighting with a light saber in the dark put humanoids at a disadvantage? Does the eye having to adjust from light to dark so quickly make it more difficult to keep track of surroundings?

[Assistant] Not much. Remember Luke's opaque helmet? A Force-sensitive lightsaber user doesn't rely much on conventional senses anyway, and a non-Force-sensitive lightsaber user is widely assumed to be of a greater threat to themselves than others.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
What gives certain sayings, phrases, or "Incantations" their power?

From things like Harry Potter's expecto pertonum, to a generic wizards with things like "Vorpo invisiupe" for invisibility, why do they need to recite these little sayings? when they can do other spells without the use of words? Moreover, couldn't i hold up a wand,(Granted if i had the powers, or if a wand is even needed) spout random nonsense and various mixed language words until something magical happened?

[Assistant]
Well, a lot of that depends on the universe we're talking about, but as a *general* rule...

So, imagine that you're a wizard. What does this mean? It means that there's a direct link between your consciousness and reality. It can be mediated through an energy field, come from outside of reality or many other things, depending on the setting. The important thing is that you can make things happen with *just your thoughts alone*.

So let's say that you want to create a fireball. How do you accomplish that? Well, it involves a variety of techniques and disciplines, but what they invariably have in common is that you're almost always going to *imagine* the effect you're trying to bring into reality.

There's a catch, however. If you start training yourself so that *imagining* fire *causes* fire... Well then, you're going to have a problem. At some point, you'll no longer be capable of imagining fire without *actually* setting things on fire.

Thus, most wizards add another step - a trigger. They train themselves so that it takes *more* than just their thoughts alone to make something happen in reality. For example, an incantation or a gesture.

These incantations are usually nonsense words or phrases you'd almost never use in any normal conversation(if you want to avoid accidentally setting things on fire, your incantation can't just be "Fire!" or whatever, it'd defeat the entire point) but are not, in and of themselves, magical. The magic is in the wizard, not in the words.

In short, incantations like that exist for one primary reason - the capability to *not* use your magic is just as important as actually using it.

Of course, depending on the universe, a lot of that can change. In some settings, wizards are effectively programming reality and the incantations are programs. In others, particular incantations are like carved pathways into reality - the universe responds to the intent of wizards so that a particular effect is associated with a particular action. Etc.

Still, the above is the general rule.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[Super Mario Bros] Is it Princess Peach or Princess Toadstool? King Bowser or King Koopa?

Is one of them a family/house name? Is one of them their birth name that they changed upon ascending to the throne? Is Peach like a nickname that only people like Mario and Toadsworth have the right to say and if some random citizen Toad addressed her as such, he'd be thrown in the stocks?

[Assistant]
He is King Koopa in the sense that he is king of the koopas. It's like "Queen Elizabeth" vs "the Queen of England"- he's King Bowser, the King Koopa.

Princess Peach Toadstool is her name. Most people call her Princess Peach (see "Princess Diana") but her family name is sometimes used for formal occasions.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [Star Wars] Why does Darth Maul hate the Jedi so much?

[Assistant]
As a child he was given to Darth Sidious, who trained him to be a Sith and told him "Jedi are bad" quite a bit. He didn't really interact with any until he went after Qui Gon in the desert.

After that, and the later battle with both Qui Gon and Obi Wan, Maul had plenty of personal reasons to hate them.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
Why does the shingles vaccine work?

My understanding of the way vaccines work is that it gives your body a chance to create antibodies so when it comes in contact with the actual virus it knows how to fight it. So, why is it that you can get shingles more than once but a vaccine exists? Wouldnt having the virus originally give you the antibodies that you would get from the vaccine?

[Assistant]
Shingles is an unusual vaccine and it's aimed at an unusual virus.

Varicella Zoster virus (VZV), the virus that causes both chicken pox and shingles, is a member of the herpesvirus family. Herpesviruses are an enormous group of viruses; virtually every vertebrate species probably has one to ten or so specific herpesviruses that infect it, and maybe it alone. These are extremely host-specific, host-adapted viruses.

Herpesviruses in general have complicate life cycles as viruses go. Typically, they'll infect they host when very young, set up a brief outright infection, and then move somewhere in the body where it can go latent (that is, more or less dormant) for the rest of the host's life. At intervals, the virus wakes up, sheds a new batch of viruses, and then goes back into latency. (There are lots of exceptions to this but this is your generic herpesvirus.)

VZV infects young children (causing chicken pox) and then moves into nerve bodies where it sets up a life-long infection, but doesn't do very much other than occasionally shed a few viruses.

Why doesn't it reactivate more often? Because the host immune system prevents it from reactivating. It's shielded from immunity in the nerve bodies, but if it tries to reactivate then host T cells (especially) and antibodies (probably less important) promptly shut it down.

Older people tend to have less potent immune systems than young people, and if the virus hasn't been reactivating much over the years, then their immune system may not have seen any virus for 20, 30, 50 years, so it may not be highly effective. Immunity can last a long time, but it does gradually fade away over time. If the last time your body saw VZV was fifty years ago when you had chicken pox, then it's not going to have a highly potent immune response to the virus any more.

You can get a combination of an older person with a slightly less effective immune system, and a long time since the VZV response was triggered, and you end up with an ineffective immune response that allows the virus to reactivate and cause shingles.

(It's been suggested that because chicken pox vaccine has been so effective, older adults are not getting re-exposed to children with chicken pox as they used to, so they aren't getting regular immune boosts against VZV, and so they're more likely to have shingles reactivations. Not sure this has been carefully demonstrated, but it seems reasonable.)

So you can see why getting a vaccine would help. It's a way of harmlessly reminding your immune system about this virus that's deep in your nerve cells, hidden from your immune system, and re-charging the specific responses.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: How come in aquariums the predators dont gobble up the prey? Are they trained not to do so?

[Assistant]
Many fish will only attack when they are hungry, or won't attack fish they the don't recognize as food.

A good aquarium will be curated to take both these into account, and will only contain well fed, mutually compatible species.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[DC] How do Flashes villains stand a chance against him?

Like, Flash is faster than Superman, who is faster than light. Why does his rogues gallery include a mind-powers Gorilla, a joker rip off, and some asshole with a freeze gun? How do any of Flashes villains other than Reverse Flash pose a threat to him?

Heck, thr first Barry Allen story had him fight a man called, "The Turtle" or "Turtleman"(they alternated between those names), who was advertised as, "The Slowest man in the world". That's like, if I made a Flash PARODY.

So, why does Flash fight the people he usually fight?

[Assistant]
Flash limits his speed on purpose when fighting normal people, so he can stay "human". In Kingdom Come, (not sure if it's Barry Allen), he stops limiting himself and basically zips around 24/7 stopping every crime, but he no longer has any relationships with anyone else and is just a force of nature. That's why Flash jobs so much, he's trying to keep his humanity.

Marvel is about people trying to be superheroes, DC is about superheroes trying to be people. Flash is no different.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: What is anomie? [philosophy]

[Assistant]
It's a state when you are given a goal to achieve (e.g. get cake) but no guidance as to how to achieve it. Moreover everything you try turns out to be somewhat "wrong" and not accepted by other people but noone can tell you how to do it right.

Anomie makes people upset and they need to figure out "a way" to achieve goals. For example people may withdraw themselves from life (*cake is bad*), choose to achieve set goals in a way disapproved by the society (get cake in any way possible whether others like it or not) or rebel (*I don't need cake*).


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[LOTR: The Two Towers] Thoden listens to Aragorn and Gandalf and rides out to meet Saruman's army instead of evacuating to Helm's Deep. What changes?

In the movie, Aragorn and, more importantly Gandalf, urge Theoden to take the fight to Saruman head on. Instead, Theoden chooses to evacuate his people to the safety of Helms Deep and weather a siege instead of risking open battle. Gandalf especially seems to believe this is a foolish choice and rides out in a huff to go find Eomer and bring him and the Rohirim back to fight. \*But\*didnt Theoden make the right call? Had the remaining men ridden out to fight Sarumans orcs and Urukai in battle, while they would have maybe had an advantage in mobility (depending on how many warg riders there were), they would have been vastly outnumbered and Edoras would be completely unprotected. They would have been even more outnumbered because the old men and young boys who fought at the siege would have been next to worthless in mounted combat. Edoras is a hillfort with very few defenses for the main hall, let alone the farms and villages around it. So even if the main part of Sarumans forces engaged in the main battle, a small contingent could easily destroy the city and kill all the civilians. And had Theoden sent the women and children out to Helms Deep while taking his army to meet the orcs head on, then the warg riders would have slaughtered the civilians on route. Gandalf still would have taken the same amount of time to reach Eomer and bring him back, so wouldnt have tipped the balance any more than it did in the actual battle. The men still might have won the battle, but it would have been at the cost the lives of most or all of the women and children. Gandalf acts so self-righteous and pissed off when Theoden decides to evacuate to Helms Deep, but it seems to me that Helms Deep was the only option Theoden had where he had both a chance of winning the battle and saving most of his people.

Edit: I'm talking specifically about the movie. I don't remember if it's different in the book. Also, I forgot about the elves. But I don't see how their presence on an open battle field would change the outcome anymore than they did at the siege.

[Assistant]
Evacuating to Helms Deep provided one single concentrated point where all of the military strength Theoden could muster was located and it made Saruman's best course of action a simple and direct and overwhelming strike at the point to eliminate Rohan's leadership and available military assets in the region making an organised defence next to impossible. But if Theoden had gathered an army at Edoras he could have amassed a force similar in size to the army he took to the Battle of the Pelennor Fields and sending out patrols and detached companies (like the one Eomer lead earlier in the film) from Edoras to patrol and skirmish with uruk-hai raiders to protect the villages most of the population lived in, utilising the wide open spaces and the Rohirrim's greater mobility to generally win such battles. That generally would have made it an attritional war of movement that would have worn down Saruman's forces over time, but it would meant more raids and destroyed farms and villages, and more casualties among the civilian population and a stready stream of battle casualties of the length of the war. Saruman might have tried to directly attack Edoras (or another important castle or town) with a large army but that army would have been intercepted en route and either harassed or engaged (with similar or greater numbers) in terrain the Rohirrim are superbly well suited to fight on.

Generally speaking fighting out in the open more aggressively would have been a more certain outcome for the war but would have taken a greater toll on Rohan while defending Helms Deep invited a risky battle that would have decided the war quickly while surrendering the initiative to Saruman entirely, if he wanted to Saruman could have set up fortifications on the roads to Helms Deep and starved out the fortress while the bulk of his army ravaged the rest of the country.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [DC] Why was it so easy for Kryptonians to die out? Did they not have a single colony or offworld diaspora?

[Assistant]
Essentially they didn't. Justifications varied over various editions, but ultimately Kryptonians basically decided that they would be better off relatively isolated on their own world, being all enlightened and advanced with crystal magic and stuff.

Unless you want to count Daxamites as sort of kryptonians, in that case they did have one other world.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
What mathematical language did Gerardus Mercator use to describe his projection?

In the Mercator projection, the y-position of a coordinate is given by the log of the tangent of its latitude. This was laid down in the 1500s. The concept of using functions to describe geometry came a bit later with Decartes, and the logarithm wasn't described until the next century either.

So what tools or language did Mercator use to describe how coordinates on his map could be constructed?

[Assistant]
Surprisingly, none. There was no talk of functions. What Mercator did was just make sure lines of longitude and latitude were straight, at right angles, and that (at small scales) distances worked the same in all directions -- i.e., that if 1 inch north was 10 miles north then 1 inch east had to represent 10 miles east.

This combination of constraints made it so features at high latitudes were depicted much larger than those near the equator, since distances between lines of longitude were smaller as you got closer to the poles and he's drawing them as equally distant everywhere. The precise way the longitude lines get closer near the poles corresponds to the spherical curvature of the earth and if you work out the math of how spherical coordinates get transferred to this style of map you end up with the log of the tangent.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: Why does facebook need two apps, one for messenger and one for general use?

[Assistant]
Because messaging used to be a secondary product. The primary Facebook usage used to be the status updates, commenting and the stuff. People were active Facebook users who went to a website, entered their passwords and logged in, played games - and then, used the messaging function at the bottom right. The rise of IM services like WhatsApp made people realize that it was far easier to just message without jumping through all those hoops. Not to mention, Facebook isn't as exciting or popular as it was 5 years ago. It isn't an organic network of friends like it said on the cover. It is mostly an RSS feed of your favorite bands and shows and people. People don't post anymore. Nobody is going to all that trouble of logging into the entire Facebook (both the website and the app) just for messaging which is just text.

The competition started including additional features like stickers, calls which was not possible to include inside the Facebook app. So they made the smart choice of making it a seperate app that allows them to use it as a distinct developement platform for making bots, turning into a money transfer service, and a formidable communications app in itself. One tap and you cut directly to the chats. No noise like status updates and comments here.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[Warhammer 40K] Have any genestealers ever managed to infect an ork?

Would all of its spores carry genestealer DNA? What if the genestealers made their own genestealer cult go on a WAAAGH?

[Assistant]
Apparently yes, since Kryptmann was able to sic the Tyranids on the Orks with the use of Genestealers.

But other fluff says that Ork genestealers don't see much "success" thanks to the Ork's inherent aversion to anything "un-orky".  More interestingly, the few Ork genestealer broods that survive long enough to reach critical mass are by necessity so "orky" that they rebel against the hivemind when it arrives.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[Hitman Blood Money]Why are all the White House surveillance tied to one terminal and videotape?

In the level amendment XXV where 47 assassinates the VP inside the White House, all the surveillance cameras in the building seem to be tied to a single computer and tape near the entrance of the level. You need to get this tape to get a good rating. What kind if security is it where the camera's are all tied to a single computer and tape especially since it's the White House?

[Assistant]
It's not; most of the levels in the game are recounts told to the reporter by the former head of the FBI and current head of the Franchise; he's embellishing the tale about the circumstances of 47's death and in some cases is blatantly lying. *

Alternatively, it's due to the villain's plan; by allowing the President to be killed as a result of lackluster security, they could use it as part of a political attack or scheme to make the vice president look better. If they left a record of the Vice President pointing out security failings, they could use it as part of a smear campaign to add a pro-Franchise head of security in the current head's place to spy on the rest of the staff.

Edit: Also, the levels allow for any number of ways for 47 to kill his targets but according to official canon, the real way the missions go down are through Silent Assassin runs - all of 47's kills are registered accidents, leaving no trace he was ever there.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] Is it possible for a moon to orbit inside the atmosphere of a gas giant?

[Assistant] No, for two main reasons. The tidal forces near the gas giant would be great enough to disintegrate the moon (the Roche limit). Even if this weren't an issue, the drag forces from the gas acting on the moon would cause its orbit to decay (this is why the ISS needs to keep re-raising its orbit).


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[MCU]Why is Peter still broke despite his father figure being a literal billionaire?

There's no real reason why Tony wouldn't want to help him and May out. Peter clearly means more to him than a regular business partner, he mourned his death more than anyone but he doesn't have the decency to churn out some pocket money from his billions to help him.

[Assistant]
Who is to say he isn't helping them out?

May doesn't appear to have a job outside of taking care of peter and working with her charity.

Its likely that Stark bought out their property, car, tuition so that they could both focus on their lives while only having to pay for incidentals like food/clothes/etc.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
When your arm falls asleep while sleeping... Is there any biological mechanism that prevents complete loss of vascular/neurological function?

After sleeping on a plane the other day, the position I was in caused my arm to completely fall asleep, I mean completely numb. It took about 5 minutes to regain feeling.

Now, I know this sensation is due to obstructed blood flow and/or pinched nerves, but is there any sort of 'built in' mechanism that would wake someone up before a limb goes "dead"? Is it even possible to happen?

Basically: is there an instinctual twitch or synapse that causes one to move a limb before complete blood flow stops?

[Assistant]
Doesn't really seem to happen - no. And this is usually caused by a pinched nerve. The pressure in the arteries should be high enough that there would be a HUGE difference in arm pressure between that necessary to pinch a nerve, and that necessary to cut off blood flow.

When the nerve begins to get pinched, the pressure starts to block nerves selectively. The motor axons stop conducting first, then the sensory axons, then the pain axons last.

Normally the sensory axon activity inhibits pain axon activity to prevent heavy tactile stimuli from being perceived as painful.

When your nerve gets really pinched, and all normal sensory axons are blocked, but pain axons are not, all stimuli evoke burning pain - warm, cold, touch, etc, even when they are not causing tissue damage.

So the fact that EVERY somatosensory stimulus evoked pain SHOULD wake you up, but in practice it doesn't really work that way - but this explanation should give some understanding to people with sciatica, back pain, or ulnar nerve entrapment.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[Star Wars] The Rebels and Empire couldn't operate their normal fighters on Hoth due to the cold, but how is Hoth colder than the vacuum of space?

Outside of being close to a star, space has an average temperature just slightly above absolute zero. So how could the Rebel and Empire fighters operate in space but Hoth is somehow too cold for them requiring the Rebels to use snowspeeders?

[Assistant]
Space is technically cold, but vacuum is a really great insulator.   Most ships actually have to figure out ways to dump heat in space, rather than figure ways of retaining it.

That's probably what made the fighters hard to operate on Hoth in the first place.  They're designed to give up their heat easily, and thus Hoth's atmosphere quickly cooled the ships below operational thresholds.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[Futurama] Why did Fry wake up 6-8 hours early?

Phillip J. Fry was frozen at exactly 12:00:02 am on Jan 1, 2000 for 1000 years and yet he wake up around 8-9 am in Dec 31, 2999.

Where are those missing hours?

[Assistant] The pod wasn't waiting for a specific time, it was counting down seconds. Each day consists of 84600 seconds. There are 365.2422 days in a year, on average, including leap years. That means that there are 31,556,926,080 seconds in a thousand years; then, using a simple timestamp conversion tool, we find that that many seconds after 12:00:02 am on Jan 1, 2000 is... 4:48 am, December 31, 2999. In fact, the error is in the other direction by a few hours, which we can probably attribute to the thawing process being somewhat slow.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
When something is x light years from earth, are those years measured from the point of view of us or the light?

Yesterday, I read up on Special Relativity and it's raised more questions that it answered.

If I understand it correctly, then my calculation shows that, if something is travelling at xc (with x being between 0 and 1), then the time that passes for an observer is 1/(x+1)^-2 times the time that passes on the object.

What I don't understand, though, is that if something is travelling at xc away from us, then we are travelling at xc away from it. So where does time pass slowest? On earth or on that object?

Edit for clarification of Title question:

For the question in the title, I mean the following: Time moves slower, the closer you get to the speed of light (c). If a star is said to be 4 light years from earth, are those 4 years measured as time passes on earth, or as time passes for each photon of light that is travelling that distance from the star to earth?

[Assistant] In a very non-rigorous sense, time doesn't really pass for a photon. Those 4 years are from the Earth's frame. It'd be basically the same from the frame of almost any astronomical object, because at typical interstellar/intergalactic speeds (hundreds of km/s), relativistic effects are still pretty small.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: Why do a lot of artists paint portraits upside down?

[Assistant] It's so you have to see what's really there and not draw from preconceived notions like when you first learn how to draw as a kid. For example, a nose really is just shading-- no solid lines exist in reality.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] The electromagnetic field is one where the electric field is 0 but the The Higgs field has a non-0 value everywhere. What does that mean? It's an energy field with constant charge?

[Assistant]
It's simpler than that.

The the level of water on a swimming pool. This is a field, it's a number that takes some value everywhere on the lake. If you have a windy day, this number can change very dramatically because you can have waves forming, but if you average out the waves, you have something which is the expectation value of the field, which is the level of the pool on a sunny day with no wind or kids jumping around.

You can change the level of the pool by opening a hose on it and letting water flow. In a world without friction, if you just open and close the water source in a tiny fraction of seconds, you will a localized flow of water propagating through the swimming pool. And if you have a drain, this flow of water can just propagate until it reaches the drain and is reabsorbed.

The electric field has a 0 expectation value, which means that it corresponds to an empty swimming pool. Whenever you wiggle around a charged particle, it's as if you opened a hose on the field, you create a localized excitation (the photon), and it travels until it is absorbed somewhere else.

The Higgs field is like a swimming pool filled with water. You can create a localized excitation on top of it, but if you average out these excitations, the expectation value is still non-0.

So yes, the vacuum of the universe is filled with this invisible matter that is constantly interacting with everything that is.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
ELI5: How does software talk to hardware?

I get how GUI software communicates with the Operating System, but I don't understand how the OS and drivers etc actually work with hardware.
For example, how is information regarding the pixels we see on display decided on and passed around until it reaches a monitor?
I guess I'm asking an answerable version of the chicken/egg thing. What information starts where, where does it go, and how does it get there?
I hope this makes sense.

~EDIT~

Some fantastic explanations here guys thank you so much. In the process here I learnt the meanings of a fair few terminologies and concepts that I had taken for granted for years and tied together a few bits of bobs of information I had in my head.
Much appreciation. Many wow. Thanks.

~EDIT THE 2ND~

You wouldn't believe I study networking in my spare time. I guess, having ready your answers, I could have dissected, metaphorically, the components of a computer and logic'ed my way through! But I would not have ended up at the much clearer picture I have now!

[Assistant]
Computers use a thing called Memory Mapped IO. Basically, the hardware makes itself look like just another chunk of memory so the CPU can read from and write to it.

Most of the hardware systems are not connected directly to the the CPU. They hang off of the bus. The bus is effectively a set of wires that a bunch of devices all connect to. Each device on the bus has an address and there is some way to flip between reading and writing.

So, to put pixels on a screen, the CPU would write the pixel color and coordinates into the bus buffer, set the address of the graphics card and tell it to write. The GPU would get that and do whatever it does to put images on the screen.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [MCU] Since the Sokovia accords banned AI, how does Stark justify his continued use of AIs in his Iron Man suits (as well as War Machine's and Spiderman's suits)?

[Assistant]
JARVIS, FRIDAY, KAREN etc... aren't the same type of AI as Ultron was. They had certain protocols that they had to follow even if they lost their memory banks. Ultron on the other hand, was a way more advanced type of AI, he was able to think freely and independently.

And honestly from what we've seen so far, JARVIS and FRIDAY seem to be like a live-in butler in your computer, your phone and various other appliances.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
So how *does* quantum computing work?

I've read a few vague descriptions of what quantum computers are capable of, but not really anything about working with them. Eventually, when we've got these things, writers of those programming books for bare, bare beginners (just throwing that out as an example) will need to be able to explain their workings simply.

So I've been pondering lately, and I think I've begun to get a handle on how they work. What I understand of them has gotten me very excited, but my understanding of them is based on gleaned knowledge.

As far as I'm aware: **EDIT: I was dead wrong, read the comments for real science!**

1. Quantum computing relies on being able to "choose" one superimposed state over another based on arbitrary criteria. This might be seen as akin to the cat in Schrodinger's box clawing its way out. What happens when more than one version of the cat wants out, I have no idea (a random one wins, I'm sure). Is there a way to compare a number between two superpositions and 'legitimize' the superposition with the larger value?

2. Nothing stops you from putting a "Schrodinger's cat box" inside another "Schrodinger's cat box". You can compound the effect recursively. Yes?

With two and one above together, you can make a binary tree of "meta-Schrodinger boxes" with a qubit at each branch. You could test an *astronomical* number of superpositions against each other using whatever fitness number you see fit.

So a quantum computer would be analogous to a genetic algorithm, except that instead of randomizing gene variables each generation, you test *every possible variant at the same time* and return the best one in nearly constant time.

Deterministic, complete information games would be unbeatable if you can come up with a proper way to generate a fitness numbers--a computer could play every permutation of a game of chess or go.

And such things as getting bipedal robots to walk would be trivial (if a bit uncanny valley) if the program understands physics and its own weight and capabilities--it could calculate every little twitch.

If I'm dead wrong, thanks for reading this far, at least. How would a quantum computer *really* work, and how would one go about actually programming one?

[Assistant]
Please for the love of god don't try and derive the original science from a something that was originally meant to show how ridiculous QM is.

Quantum computers work by getting a superposition of values entangled into a quantum register, and then applying various gates and transforms to knock out the states that are not needed, and then the quantum register is measured/"read out", and this is repeated and repeated and repeated until enough trials are done that the distribution function gives up the answer you're interested in up to a certainty.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
Is the ratio between females and males similar between all primates?

Particularly interesting would be a) at birth and b) at "breeding age" (don't know how you say for people).

**Edit:** To clarify, I mean a ratio in the number of specimens, and I'm talking about the few-percent difference that I've heard about in humans.

[Assistant]
There's a good theoretical argument why the male/female ratio should be close to 50/50 in any species.

Suppose the ratio were different, let's say 9 females to 1 male. Imagine that a random mutation caused one of the genders to be more probable to be born than the other. That mutation would spread through the gene pool, increasing the ratio.

In that scenario, the most increase the females could get from a mutation would be from 0.9 to 1.0, a 10% gain, while males could go from 0.1 to 1.0, a 1000% gain. A random change will always benefit the gender with fewer representatives, which drives the ratio to 50/50.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
ELI5:What is plasma? How is it considered a fourth phase of matter?

As a followup question, is fire plasma?

[Assistant]
Suppose you start with an ice cube.

In a solid, all the atoms are packed pretty rigidly together.  They vibrate around but don't move much.  In your ice cube, the atoms actually form a lattice, a repeating pattern.  Not all solids do this (amorphous solids have no repeating pattern).  Solids have a definite shape and volume.  If you put your ice cube on a table, in a room cold enough that it doesn't melt, it'll just sit there.  It keeps its shape and it doesn't change size.

If you add some energy to your ice cube, you can melt it.  Then you have a liquid.

In a liquid, the atoms have much more freedom of motion.  They still like to stick close to each other, but they slip and slide around.  Liquids have a definite volume (like solids) but no definite shape (this is new) - they take the shape of their container.  Melt your ice cube on that table and the water will spread out, attempting to cover the whole table in a flat puddle.  Unless you have a very small table, or an impressively large ice cube, it won't be able to.  Liquids do not change their volume.  Put that same ice cube in a cup and melt it, and the water will conform to the shape of the cup (and be a cylinder, instead of the flat puddle on the table).

Take that puddle of water and add even more energy.  You'll vaporize it.  Now you'll have a gas.

In a gas, atoms have even more freedom of motion.  They also no longer feel the need to stick close to each other.  Gases have neither definite shape (like liquids) nor definite volume (this is new) - they will expand to fill their container.  So that puddle of water on your table, in gas form, now spreads out to boundaries of the whole room (it's important to note the definition of volume, though.  Throughout all this melting, the number of atoms hasn't changed.  So if you cooled the room back down, it wouldn't be filled with ice - you'd have the same size ice cube as you started with, if you somehow were able to shepherd all the gaseous atoms back together.  What has happened is that the water vapor has spread out across the whole room, instead of staying concentrated in one part of the room like the ice cube).

Now take that water vapor and add even more energy, enough to ionize it.

Now you finally have plasma.  A plasma is similar to a gas, but even more free.  In a plasma, you have ionized the atoms of the gas.  Now instead of being a cloud of atoms, you have a cloud of positively and negatively charged particles (you broke the atoms apart into electrons and chunks of the nucleus.  The exact size and composition of these chunks depends on the substance and how much energy you added).  Since plasmas are composed of charged particles (which are called ions, which is why the process of making them is called ionization), they'll react to an externally applied electric or magnetic field.  Exactly how they react depends on the nature of the field you apply and some other things.  Plasmas have no definite shape or volume (like gases), but can form structures when subjected to certain fields (this is new - if you google plasma globes you'll see examples of filamentation, one of the shapes a plasma can be induced to take).

At each stage of the game here we have added energy and drastically altered the nature of the substance we were working with.  This is how we distinguish the phases of matter, and how plasma is the fourth.  We went from atoms packed rigidly together, to atoms that slip and slide past each other but don't get too far away, to atoms that just fly around anywhere, to atoms that have been broken up into smaller bits.  You can, in fact, keep adding energy and get to other, more exotic states of matter (if you add enough energy, you can break apart the neutrons and protons in the nuclei and get quark-gluon plasma.  Today this only happens in huge particle accelerators for brief instants, because the amount of energy required is.....very big.  But as the universe was cooling down after the Big Bang, there was a time where all matter was this).  And conversely, you can take energy away from solids (cool them down) and get more phases (these are the Bose-Einstein condensates.  In these, all the atoms kind of lump together into one super atom and you get a variety of quantum effects usually only seen with much smaller particles).  Then there are theoretical phases that haven't been observed yet, like color superconductors and all kinds of neat stuff.

The bottom line is, a phase is a general state of matter where it behaves in a certain way.  Solids, liquids, and gases are the three phases that a person is most likely to run into in everyday life.  Plasma is another state of matter, often called the fourth state of matter because it is also not that hard to observe in basic settings.  Plasma is characterized by the ionization of constituent particles, and otherwise behaves much like a gas.

To answer your followup, yes.  Well, depending on whether or not you are a chemist either yes, or no, fire is a chemical reaction but flames are a plasma.  When you burn something, you are releasing a lot of energy, and turning whatever you burned into a gas.  If the fire is hot enough, the gas will ionize and you have yourself a plasma (where 'hot enough' is defined relative to what you are burning.  Different things ionize at different temperatures).


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: why are some guns more deadly than others? Is it something to do with the speed at which bullets come out, or the size of the bullet? Like why is a revolver taken to be a stronger weapon than a glock/pistol?

[Assistant]
Oh dear, you're already getting a lot of terrible information, so let me help you out:

There's a lot of different factors that determine how "deadly" a firearm is.  The most important is probably the type of round it fires.  The keys to a deadly round are how much force it carries, and how effectively it transfers that force to the target.

There's two major factors in a firearm cartridge to consider: size of the bullet, and size of the casing.  Bigger (wider/longer) bullets will obviously carry more force simply by virtue of having more mass.  Longer casings will provide the bullet with more velocity because they hold more gunpowder.  Generally speaking, a higher combination of mass and velocity is more deadly because it carries more force to the target.  However, that only gets us part of the way there.

How that force is delivered also makes a difference.  The goal of a bullet is not to simply penetrate a target, but to damage it.  In this case, shape and composition of the bullet come into play.  Rounds that have more solid bullets (such as full metal jacket) and/or more pointed ends are more likely to cleanly penetrate a target.  Cleaner penetration means the shock will be concentrated to a smaller area.  Also, if the bullet passes *through* the target, then some of its energy will not have been transferred.

Rounds that have flatter/softer tips, on the other hand (such as soft-point or hollow-point) will expand on impact, increasing the area in which the force is delivered in the body.  This means force is delivered to a wider area, causing greater shock to the target.  The expansion also decreases the likelihood that the bullet will escape the body, ensuring that *all* of its kinetic energy is transferred.

So, generally speaking, you want a heavy, fast (but not so fast that it over penetrates) round with a soft, expanding tip.  Right?  Well...maybe.  Another important fact about firearms is that they are very situational.  They are a form of tool, and there's a right tool for every job.  While you might want a medium-velocity hollow-point round to shoot a regular target, what about someone behind cover?  In that case, a more pointed round (such as a spitzer bullet) with a higher velocity would be better.  What if you're shooting at very close range, possibly with poor visibility?  You may not want a bullet at all, but rather a shell loaded with shot pellets.

As for the firearms themselves, it's also very situational.  Most people would probably pick a modern automatic or semi-automatic rifle as "more/most deadly," but it depends on the use.  If your target is very far away, you might hit them with a bolt-action rifle when your AR-15 can't even fire that far.  If they are close, a shotgun would be more effective, while a rifle round would probably fly right through them.  Infinite other questions also affect the answer.  Is the target shooting at you?  Are they wearing armor?  What is the terrain/climate like?  Is being immobile an option?  Is the target aware of your presence?

An interesting example comes from the war in Afghanistan.  The standard issue rifle for the US military is the AR-15 (M16).  This has long been considered the modern gold standard for military rifles.  However, in Afghanistan, Taliban fighters were using less fancy rifles, but with higher-power rounds.  Due to the open terrain of the country, they could take up position far away and shoot at US soldiers, while the AR-15s didn't have the range to reach them back.  The military then started re-issuing the M14.  Although it's an outdated firearm (taken out of service in 1964), it fired a round that could travel the distance and properly engage the Taliban.

TL;DR: Mass + velocity are generally good, but otherwise everything is situational.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[General] Why no anti-zombie armor?

Seems like most people end up getting bit in the neck or the arm, etc. Doesn't seem like it would be too difficult to get a scarf and a couple layers of clothing at the least.
Something moderate would be like tying together plastic (think packaging stuff that takes a damn knife to open) sheets together with string to form mail.

Military/police with riot gear should be all but impervious except for being mobbed and literally crushed by the weight of 20 people.

[Assistant] They do use it in World War Z (the book, not the movie) it's a combination of Kevlar and some other classified material.  It includes full body armor, boots, attachable gloves and a hood that covers your whole face.  All bite proof.  That's not to say that they're totally invulnerable inside the suits, they were attacked by feral cats, dogs, kids, and even lone survivors who had gone nuts or didn't know the military was friendly.  For zombies though, once you figure out a strategy that beats them, they don't adapt.  They're one-trick ponies and once you've worked out how to beat them, repeat until every single one of them is dead.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
Do we know what the Earth was like prior to the asteroid collision that formed the Moon?  Could complex life have lived here?

I've read the Earth was still molten, but I don't know if that's the popular consensus or more of a guess.  I was wondering how likely it would have been for complex life to have lived here pre-Moon before getting being wiped out by the asteroid.

[Assistant]
It's highly unlikely that there was life. The collision happed several hundred million years after the solar system formed. The Earth, at that point, couldn't have supported life.

Also it wasn't an asteroid that hit the Earth, it was a planet the size of Mars.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
How were we able to measure how big the sun is?

I was just wondering because I've heard throughout my schoolyears the size of the sun and the distance, but we are not able to get close to it. How do we know?

[Assistant]
Combination of two methods for direct observation:

1. Parallax: gives you the distance to an object. Works by making angular measurements of the distant object from two positions whose location is precisely known, and then using trigonometry to determine the distance. If you know two angles and a the length of one side of a triangle, you can figure out all the remaining dimensions.

2. Apparent width: Using the appropriate filter (welding goggles work well) one can measure how wide an object appears at a certain distance from ones eye. Once you know the distance to the object, you can determine how wide it really is by taking the ratio of distance and widths. (real width) = (measured width) x (distance to object - distance to measured width location) / (distance to measured width location). Of course in reality, the sun does not have a defined surface or edge, but this method will get you some idea of the scale of it.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[Harry Potter] What happens if someone livestreams video of a Basilisk?

Someone manages to get one looking at a camera and streams it to the whole world. Would anyone watching just get petrified? Or would the magic lose its potency over the Internet due to latency issues?

Edit: Thanks for all the answers so far, everyone! Love this sub.

[Assistant]
Colin Creevey, being the kind of muggle-born who would try this sort of thing, actually attempted to take a picture of a basilisk with a camera once. The result was a dead camera fried from the inside out and a petrified Creevey due to his sight through the viewfinder.

Nothing can really record the image of a basilisk's eyes without frying.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[40k] Can space marines quit?

Say Leonos has seen enough, he saw his best friend and battle brother of two centuries get eaten whole by a foul Nurgle daemon, and then shit out promptly, instantly being transformed into a daemon of Nurgle himself in the process; that a warrior of the Emperor true of heart and of fearsome quality could so instantly be overcome, and converted almost trivially into the foulest evil which he had previously so hated has ruined Leonos' ability to carry on. "I'm not a heretic" he swears up and down to his chapter master, he just wants to walk out of the fortress monastery and take up farming. He's seen enough and he wants to hang up the old boltgun.

What actually would happen in this case?

[Assistant]
For most chapters, no; a Marine who would turn away from his commitment to eternal war would be suspected of some manner of corruption. The indoctrination of a Space Marine is as much a part of their armor as their exocarapace, and cracks in said armor are a massive liability. In many cases, it would be in everyone's best interest to quietly take them out back and shoot them.

There is at least one exception. Among the Salamanders there is a tradition called the Burning Walk. A solitary journey into a vast desert, traditionally undertaken by those too physically crippled to go on as Marines. It is normally regarded as a voluntary (but honorable) death sentence. If something troubled a Salamander so badly that they didn't feel they could go on as Marines, they could go on the Burning Walk. Occasionally, they come back, having seen or found something that put them back on the path. Most don't come back.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
[Marvel] Electro might be one of the most powerful humans on the planet. He controls one of the four fundamental forces of nature. So why is he just this low-tier bank robber?

I don't get it, he has such incredible power and potential but seems like a goon. Is he really just... dumb?

Also, his mask is super dumb.

[Assistant] Generally electro is limited to lightning bolts and electric teleportation from what I've seen. He might control something fundamental but his control is limited, when compared to the types of super people around him he isn't so great.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: Why are ingrown toenails so common but not ingrown fingernails?

[Assistant]
Because shoes and wrong nail clipping techniques.

A toenail is easier to clip wrong If you apply fingernail techniques to it. And on top of that wearing shoes that are a bit too small can also cause ingrown toenails. On you fingers however there's no shell to cause fingernails to have to grow into the skin.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] Does it require more, or less energy to climb stairs two steps at a time as opposed to one?

[Assistant]
From a physics standpoint as long as you are going up to the same height you are always doing the exact same amount of work.

If you lift one pound ten feet in 10 seconds or in 10 minutes you still do the same work.

From a physiological standpoint it might be different.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
CMV: 5G is not Dangerous

I've seen so many people making conspiracy theories and claiming 5G is dangerous, and I really don't believe it. Ok, I understand it's high frequency (Gigabit speeds) radio waves, but so is WiFi, and how many homes and businesses nowadays have either 1, 5, or even 10 gigabit internet packages with WiFi. I'll admit, I'm not an expert, there may be a difference between WiFi and cell towers, but I don't see why they'd be that different. I mean I'm sure everyone has a router in their house that's somewhere between 120 Mbps and 1Gbps and I don't think we've seen a spike in radiation based illnesses since gigabit wifi became the norm. Like I said, if you know something that I don't or something that I'm wrong about tell me. I mean I am an Information Systems major but I haven't taken data communications yet (I'm supposed to take that next semester) so basically all my views are based on personal research, experience, and opinions.

[Assistant] 5G probably isn't physically dangerous, but there are privacy issues that could create a danger.  The towers have much less range, which means that by a single tower connection alone your location is known more precisely.  The bulk of 5G tech is made by Huawei, who is suspected to have close ties to the Chinese government, and whose equipment is involved in a number of potential spying issues.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: Although water is clear, why does it make certain stuff appear darker when wet (i.e clothes, towels, paper, etc)?

[Assistant]
The reason is not that water is not transparent enough, as another user suggested. But on the contrary, it is that the materials are slightly transparent. Let me explain. If you take, say, a sheet of paper, it seems to be quite opaque, right? However if you put this same sheet of paper under the microscope, you will see individual fibers, those little specks of cellulose, and if your magnification is high enough, you will notice that they are somewhat transparent. Not quite clear, quite cloudy actually, but still semi-transparent.

Now, you don't notice it usually when you look at the sheet of paper. But the light that falls on it gets refracted and reflected from these cellulose fibers, and gets trapped in the air cavities between them, and gets scattered, and finally reflected to your eye. So the sheet looks rather light. Same with stone (which consists of semi-transparent crystals, or sand grains), or fabric (with individual micro-fibers), or wood.

However if you make the surface wet, water will fill these little air pockets between the fibers, grains, crystals, of whatever your material is composed of. Where it was air, you'll now have water. It will change the optic properties of these surfaces: you will have less refraction, and less reflection. If you ever put a glass cup underwater you might have noticed that things made of glass are less noticeable, less visible underwater than they are in the air. Same happens here with your surface. The micro-pockets of air that used to scatter the light, making the surface lighter, don't scatter it as well anymore. The light is not sent back into you eye, but goes deeper into the material instead, and gets absorbed there. And it looks darker.

When there are no tiny pockets of air (a mirror, or a polished table) it does not get darker when wet. When the material is not semi-transparent even at a microscopic level (say, a brass knob, or a steel knife), it also doesn't get darker. And now you know why =)


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
Why is static friction stronger than kinetic friction?

Why can my cars handbrake keep it still but it cant stop it once its moving.

[Assistant]
There is no simple answer to this question, and it is a rule "in general" not a hard and fast rule.

As a rule, when an object is moving, the small imperfections on the surface get smoothed out a little bit, all facing the same direction, and thus the friction between the surfaces will decrease. If you have two smooth, clean pieces of metal, you will often find the coefficients of static and kinetic friction are the same.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] Where did the practice of (women)shaving legs originate, and why?

[Assistant]
The modern practice of women shaving their legs in western countries was largely driven by changing clothes from around the 1920's combined with aggressive marketing and advertising (aka fashion). Arm pit hair was the first to go - the first safety razor specifically aimed at women was released in 1915, the same year that an edition of Harpers Bazaar magazine came out with an issue featuring a model wearing a sleeveless dress and no hair in the armpits. It wasn't really until WW2 / the 1940's and the era of the pin-up girl that shaving legs became the fashion.

Of course in non-western cultures the practice of females removing body hair has been practised on and off for thousands of years at different times and places, for example in ancient Egypt, Greece, and Middle Eastern countries. This was done for a number of reasons, from perceived hygiene benefits to class differentiation.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
If a person loses their dominant hand, will their other hand ever gain the dexterity of the lost limb?

On a related note, if somebody is born without the hand that would otherwise be dominant due to a birth defect, will their remaining hand be clumsier than normal? Does that question even make any sense?

Thanks, experts!

[Assistant]
this is an empirical question, not a theoretical one, because dexterity is defined empirically.  and because of the limited number of people to which it applies, anecdotal evidence is appropriate.

a friend of mine had his right arm badly injured in a car accident when he was a kid.  so he had to start doing almost everything left-handed.  a decade later, he was very proficient, but you could still clearly tell that he wasn't as dexterous (even as, say, an 8 year old using their natural hand).  writing, typing, eating, you name it.  of course practice makes you much better.  but to me this is explicit evidence that one does not fully gain the dexterity of their original dominant hand.  no amount of other people who might have gotten slightly better can disprove this empirical observation.

edit: to the mods:  unless you understand why it's inappropriate to use a fixed effect statistical analysis on data of this nature, you shouldn't be removing this comment for being anecdotal.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
ELI5: How is a Grand Jury able to indict someone other than the defendant to whom they're assigned? (Planned Parenthood lawsuit)

As I understood it previously, the role of the grand jury is to determine if something is worth bringing to trial by jury. Having tried to read up on it a bit, its role, as I understand it is to:
1) determine that a crime was committed
2) determined that there is enough evidence against the defendant that a full trial is warranted and it should be explored to the level of 'beyond a reasonable doubt'

But the key words in item 2 are *against the defendant*. I could understand if it was a recommendation, but I'm curious what it is that legally empowered it to actually indict the producers of the video, who weren't actually on trial.

[Assistant] In many states, a grand jury has considerable powers. They aren't just a tool for the prosecutor to get an indictment, they are tools of the state for the purposes of justice. Sometimes, justice means indicting someone other than who the prosecutor wants you to indict.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
ELI5: What is the difference(s) between using debit and credit cards when purchasing something?

my bank card allows for both options (as I understand is common) but I never really fully grasped the difference.

[Assistant]
I think op is asking what's the difference between using his/her debit card as debut or credit when making a purchase.

When you use debit, you have to input your pin, and the money is automatically deducted as if you made a withdrawal at an ATM. When you use credit, the transaction is processed by visa/MasterCard and a preliminary hold is placed on your card for the purchase price (sometimes a little more or less depending on if you're buying gas at the pump or dining out) to verify that your card has enough available credit to handle the transaction, but the money hasn't technically left your account yet. After the hold is released (anytime between a few hours to a few days) the hold is released and the funds are drawn from your account. You can't debit more than you have in the account, whereas credit transactions will sometimes let you overdraft your account (usually costing you a bank fee). Some banks offer rewards programs for using your card as credit which you can redeem for cash back, gift cards, merchandise etc.  Also, it'd worth noting that the places you're spending money are charged different rates for running your card depending on if you use debit or credit. Most credit cards charge the business 1.5-5% of the transaction amount, whereas debit fees are capped (in the US) at $0.24 per transaction plus 0.05% of the swipe.

EDIT: changed some of the figures for rates per TYMSMNY below and some internet research :)


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: Why doesn't Nestle bottle more water in water rich or non drought suffering states instead of California?

[Assistant]
It should be pointed out that, despite the uproar, Nestle's operations are a drop in the bucket when it comes to water usage in California. A drop so small that, even if the company were to pack up and leave, it wouldn't even meagerly solve the drought problem.

The real culprit in California is the agricultural industry.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [Matrix movie 1] What would have happened if Neo (or anyone else, for that matter) would have just surprisingly taken and swallowed *both* the red and blue pills?

[Assistant]
The idea behind the blue pill is that you wake up in your bed, its probably just a regular old sleeping pill and the idea is that Morpheus & Co. Would take you home and tuck you in and everyone forgets about it.

The red pill causes a disruption that allows Morpheus & Co. to track you outside the Matrix. They use the phone to wake you up outside.

By taking both, you would fall asleep in the matrix while your real body can be tracked outside the matrix via the disruption.

Since Morpheus probably wouldn't use the phone to get you out while you are asleep, you would just lay there until the effects of the blue pill wear off.

This is assuming the machines don't detect the disruption and dump your body from the outside, then you would be dead.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
What form of numeric differentiation is this?

I needed to code up a quick check of a routine that returned the derivative of some function so I found myself doing (f(x+e)-f(x))/e as e got tiny.  So far so good. Then a coworker said that in his experience (f(x+e)-f(x-e))/2e was more accurate for e > 0 because it was symmetric.  I checked that in the limit e-->0 they returned the same derivative for simple functions.  What form of numeric differentiation is this? Is it more accurate?  Thanks!

EDIT: Lots of great answers in here. Particularly the bit about the central finite difference having no f'' term so the next leading order term scales by e^2 . Thanks y'all!

[Assistant]
There are three simple ways to approximate a derivative, with different amounts of error from the "true" derivative.

d(x) = (f(x+e) - f(x))/e is called a "forward finite difference".  Its error is proportional to e.

d(x) = (f(x) - f(x-e))/e is called a "backward finite difference".  Its error is also proportional to e.

d(x) = (f(x+e)-f(x-e))/(2 e) is called a "central finite difference".  Its error is proportional to e^2 -- which is a big improvement when e is small.

There are other schemes that use f(x+2e), f(x-2e) etc. to create schemes whose error is proportional to e^4 or better, but they're usually not worth the effort, you're better off just making e smaller.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
ELI5: How are film crews paid?

I assume grips and sound guys are paid salaries, but actors/directors are often paid incredible sums of money for their work. How are they paid that money? Is it in a lump sum after the movie is finished shooting? Or do they get paid weekly like everyone else, just in larger amounts? How do royalties/merchandise work? How much does the studio take?

[Assistant]
Im a TV accountant.  Most everyone is paid hourly.  Directors are paid by the project.  Actors can be paid a day rate, weekly rate or episodic rate.  Some jobs, accountants, costume designers, set decorators are paid a weekly on call rate.

Checks are due the Thursday after shooting (or Friday if youre on location.)

The studio doesnt take anything!  Thats crazy.

Residuals and merchandising are depending on the contract.

Payment to writers, directors and actors are usually sent to their agent who takes 10% and sends the rest to their client.  Crew checks are handed out on set or mailed.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [Star Wars] Why are droids so inaccurate with guns? They should be the best shooters in the galaxy!

[Assistant]
Because T-800s are expensive to produce.

You could make an entire army of battle droids for the price of a single Terminator. Thousands of B1s can do more damage than any one Terminator, even if a single B1 is pretty inept.

The CIS based its strategy around swarm tactics. Lots and lots of cheap, disposable soldiers they could just throw at a problem.

Clones were a quality over quantity type deal. They were far superior to droids, but took 10 years to fully develop, they needed training, they needed food and air to breathe.

The CIS had slightly better B2s, significantly better Destroyers, and the ungodly good Commando Droids when shit got serious.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
ELI5:How did they send people into space in the early days of space exploration without computers and GPS technology?

How did the technology work for space exploration when there wasnt modern computers or GPS?

[Assistant]
Well, there were computers, though they were far more primitive. Often, this meant trajectories had to be calculated by hand.

Navigation was another matter. Though GPS didn't exist, ground stations could locate space probes based on radio signals, similar to Gps. Otherwise astronauts had to use a sextant to navigate by the stars.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: What exactly is MSG and why does it have a bad repuatation in foods.

[Assistant]
MSG is mono-sodium glutamate, a salt of glutamic acid, which itself is a naturally occurring amino acid which is essential for life.

MSG has a bad reputation because it is used widely in Asian cuisine, and became an easy scapegoat for customers feeling unwell after eating Asian foods. After all, a strange white powder from a different culture with a chemical name cant be good for you, or so the logic goes.

There are countless studies which show MSG has no negative effects if consumed in moderate amounts. There are negative effects if you take 3g of MSG without food or water, but thats not realistic.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
If someone had been dead pre-zombies, would he come back to life after being bitten?

Consider any universe. World War Z, Dead Island, Walking Dead, etc. I'm basically saying that if someone died of natural causes (or unnatural, whatever) before the "patient zero" of the zombie apocalypse, would he come back to life when bitten?

[Assistant]
It depends on the type of zombification (i.e. viral, parasitic, etc) but generally, the process requires piggybacking on some of the victim's metabolism in some way to affect the rapid changes. For example, viruses rely on existing cell replication to replicate themselves, so introducing one to dead cells doesn't do much. It's analogous to turning the steering wheel on a car. It doesn't do much if the car isn't already moving.

That said, it takes a while after death for body functions to cease completely, so some infections could feasibly still take over a *recently* deceased body.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] [Doctor Who] What's in the Twelfth Doctor's browser history that he doesn't want anyone to see?

[Assistant] The sonic glasses are tied into the tardis. When he says browser history, he means "don't look at where I've just visited." He doesn't want the first doctor to see it because its his own future. He doesn't want Osgood or Bill to see it because it would reveal too much about their own personal futures (he hops around people's timelines a bit).


[System] Given the following post, respond as if you're commenting on Reddit:

[User] Does fiber simply speed up our digestive system, or do our bodies want to expel fiber quickly?

[Assistant] Fiber acts as a bulking agent, and as mentioned by HonorAmongSteves, it carries water, creating a rather substantial mass that moves through one's digestive tract. Peristaltic movements by the colon and eventually defecation are stimulated by stretch receptors present in the wall of the colon. Due to the increased bulk in the colon, stretch receptors are activated, passage is expedited, leading to a faster mouth to anus transit.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
Rust must be a major concern on seagoing ships. What technologies and techniques are used to prevent damage?

I'd assume that regular painting would make sense, but are there specific kinds of paint for boats? Are ships primarily made out of rust-resistant materials?

[Assistant] One strategy is to use a  galvanic or sacrificial anode. Simply put, this is a chunk of metal more electronegative than the hull of the ship. When properly affixed, oxidation reactions will prefer the anode to the hull, reducing rust on the ship at cost of a non-structural and easy to replace chunk of metal.


[System] Given the following post, respond as if you're commenting on Reddit:

[User] ELI5: Why i don't get tired of eating the same breakfast for 365 days, but get tired immediately if i eat the same dinner 4 days in a row.

[Assistant]
It's a matter of habit, really.

You're used to eating the same stuff, therefore you feel more comfortable doing that. But you weren't brought up to eat the same dinner everyday, so you **want** and **prefer** the variety with that meal. Your body (really the brain) is used to that cereal, glass of milk and sandwich in the morning that you've eaten for 15 years in a row almost every day so it just goes automatically and doesn't complain about it.


[System] Given the following post, respond as if you're commenting on Reddit:

[User]
What does "A causes B" mean? How can we tell that A causes B?

I am not versed in philosophy, so please try and keep the jargon and the prerequisite knowledge down to a minimum.

[Assistant]
This actually isn't all that simple of a question, as different philosophers have different ideas of what causation means. David Hume, for example, believes that we can never actually know in many cases that A causes B.

To Hume, there is some types of causation that we can know. These are "relations of ideas", with a common example for him being math. We can always know that 2 + 2 will result in 4, because we can logically show it by showing that there will be a logical contradiction if we assumed that 2 + 2 does not = 4.

On the other hand, Hume views matters of fact, which are observable things about our world, as being impossible to know causation about. We see a relation through observation between taking an aspirin and it later reducing our headache, but Hume needs a logical way to get from taking an aspirin to reducing a headache every single time in the future.

There are two ways Hume attempts to do this. The first is through "demonstrative reasoning", which is the method we used earlier to show that relations of ideas can be known. However, if we assume that we take an aspirin and we have no headache reduction, there is no logical contradiction so this approach doesn't work. The second method is through "probabilistic reasoning", in which we need to know that **the future will be like the past**. However, then we're stuck with trying to prove that the future will be like the past. This can't be done through demonstrative reasoning because there's no contradiction if we assume that the future will not be like the past, so we must try to show that the future will be like the past through probabilistic reasoning. However, in order to do so, we must already know that the future will be like the past, which to Hume results in circular reasoning.

Thus, Hume believes that it's possible to know causation when it comes to relations of ideas, but not with matters of fact. This is just a small piece of the puzzle, and Kant makes a good effort to flip this on it's head. Hope it helps.