Science has done a terrific job of answering some of the world’s most difficult questions, but certain mysteries still elude researchers. How does gravity work? Can your pet fish really predict an earthquake? Why do we yawn so much? Here’s what we don’t know and how close we are to figuring it out.

1. Why Do We Yawn?

Theories about why we yawn are as common as grumpy toddlers at nap time, but two explanations seem plausible after experimental tests. One is that yawns help cool the brain and optimize its performance. Psychologists at the State University of New York at Albany say it explains why we yawn when we’re drowsy: Like the fan in a computer, the yawn kicks in when our performance starts lagging.

But if yawns are our brains’ way of kick-starting their efficiency, why is yawning contagious? The brain-cooling camp suggests that it’s a way to maintain group vigilance and safety. When a member of a pack yawns, signaling that he is not functioning at his best, the whole group may need to yawn for a collective cognitive boost.

That’s not the only theory floating around, though. Another explanation contends that contagious yawning builds and maintains empathy between yawners. A sympathetic yawn signals an appreciation and understanding of someone else’s condition and subconsciously says, “Me too, buddy.” So which story is the accurate one? Scientists aren’t ready to declare a winner yet—they need a little time to sleep on it.

2. Why Do People Spontaneously Combust?

Here’s what we know: Humans really do spontaneously combust. One of the first people recorded to have gone up in smoke is a poor Italian knight who burst into flames after drinking strong wine in the mid–17th century. The cause of the mysterious fireworks befuddles scientists, but they’re certain that each instance is less spontaneous than it seems. Over centuries, 120 cases of spontaneous human combustion have been reported, but because most of the cases involve smokers, a common hypothesis is that an outside flame is involved. The theory is that a cigarette scorches the skin and breaks it deep enough to force body fat to seep rapidly from the wound into burning clothing; together they act like candle wax and a wick.

It’s far more probable than the competing idea—that methane gases build up in the intestines and are sparked from inside the body by a mix of enzymes. But there’s a problem with testing both theories: Researchers can’t just walk around setting people on fire. They may have found a substitute that will answer the question, though. Pig tissue combusts in a way that’s consistent with the “wick effect,” and samples are far easier to obtain. Who knew bacon would help solve the mystery of one of Spinal Tap’s drummers?

3. Why Do Placebos Work?

When a new drug enters clinical trials, researchers need a control group against which to compare its effects. Members of this group are given what they’re told is the drug but is actually a pill containing no active ingredients, a placebo. Frequently, though, the control subjects feel the drug’s effects. Or at least they say they do. What actually happens to placebo poppers is still unsettled. Some studies have found objectively measured effects that are in line with a real drug’s results. Others have found that the benefits are only subjective; patients said they felt better after taking the placebo, regardless of their actual improvement. This mixed bag of evidence could support any number of explanations. There could be an actual physiological response, Pavlovian conditioning (a patient expects to feel better after medicating), positive feelings from patient-doctor interactions, an unconscious desire to “do well” in a clinical trial, or even a natural improvement in symptoms.

Whatever the cause, pharmaceutical companies are keen to figure out the placebo effect given its potential to throw clinical trials into disarray. Real drugs often can’t compete against the effects of fakers, and about half get scrapped in late-stage trials. For the researchers who’ve spent nearly 10 years trying to bring their drugs to market, that’s a bitter pill to swallow.

4. What Was Life’s Last Universal Common Ancestor?

A whale and a bacterium or an octopus and an orchid don’t seem to have much in common, but deep down they’re all the same. Research reveals that most of life’s tiniest components, like proteins and nucleic acids, are nearly universal. The genetic code is written in the same way across all organisms. A small core of genome sequences is also similar across major branches of life’s family tree. All this suggests that every living thing made of cells can trace its lineage back to one source, a universal common ancestor.

In theory, this idea makes a lot of sense. Getting this ancestor to show up for a paternity test is tougher. Scientists estimate that the last universal common ancestor (LUCA) split into microbes and later eukaryotes (animals, plants, and the like) around 2.9 billion years ago. The fossil record from that era is scant, and by now, the genes that have traveled down the family tree have been lost, swapped, or shuffled around.

But some features of proteins and nucleic acids encoded by these genes—such as their three-dimensional structure—have been preserved throughout time. A survey of these molecular traits offers a glimpse at what the last universal common ancestor might have looked like. Researchers have found that tiny organelles (specialized subparts of cells) as well as their associated enzymes are shared by all major branches of life, meaning that they must have been present in the last universal common ancestor. This and other evidence suggests that the LUCA was as complex as a modern cell—which doesn’t make our forebear all that visually impressive. But on the plus side, until scientists get to the bottom of this question, we can all save money on Father’s Day cards for the granddaddy of all life on Earth.

5. How Does Memory Work?

For a long time, neuroscientists thought a memory was stored in a scattered group of neurons in either the hippocampus or in the neocortex. Last year, researchers at MIT proved that theory for the first time by causing mice to remember or forget an event by activating or deactivating the associated neurons.

It’s an essential piece to the puzzle, but to recall a memory on its own, the brain has to activate the correct assortment of neurons. And how exactly the brain pulls off that trick isn’t fully understood. Studies on rodents and brain imaging in people suggests that some of the same neurons that the original experience affected are involved. In other words, remembering something may not just be a matter of grabbing it from its storage space but re-forming the memory each time it’s pulled out.

6. Can Animals Really Predict Earthquakes?

The idea that our furry and feathered friends could warn us about impending doom is a nice one, but it’s been hard for scientists to prove. Pet owners have noted how their animals acted funny just before an earthquake since the days of ancient Greece. There’s no shortage of reports, but almost every one is anecdotal, based on opinions of what’s “normal” and “funny” for an animal. And the stories are generally reported long after the fact.

It’s not out of the question that animals may sense and react to some environmental change that we don’t notice—anything from seismic waves to changes in electric or magnetic fields. However, it’s not clear that earthquakes even produce such precursors. Plus, whatever the proposed cause, it’s nearly impossible to test. If we can’t predict earthquakes, we don’t know when to observe animals, and it’s even more difficult for researchers trying to reproduce the experiment later. The few “lucky” cases where quakes happened during animal experiments provide conflicting evidence. If you’re going to rely on a cat for earthquake advice, consult one with a degree in seismology.

7. How Do Organs Know When to Stop Growing?

Every mammal starts out as a single cell before growing into trillions of them. Usually, there’s tight control over the number and size of cells, tissue, and organs, but sometimes things go very wrong, resulting in anything from cancer to a leg that’s larger than its partner. So what’s sending the “stop growing” signal?

Four proteins that make up the core of what’s known as the Salvador-Warts-Hippo signaling pathway appear to help regulate growth for a number of organs. Shutoff signals sent down the pathway deactivate the protein that promotes growth, but that’s where scientists’ knowledge stops. Where these signals originate and which other elements are affecting SWH is unknown. Scientists continue to learn how to manipulate the pathway, discovering new triggers and working their way to the source, but there are still a lot of mysteries—including how we may be able to “turn off” cancer.

8. Are There Human Pheromones?

Can you actually smell someone’s fear? Or sniff out a rat? Plenty of animals communicate with chemical signals called pheromones, but whether humans are part of that club is a contentious issue. There’s some evidence of people making behavioral and physical changes in response to chemosignals, but scientists haven’t been able to figure out which chemicals trigger these responses. And despite what the labels on pheromone-infused colognes and hair gels will tell you, no compound has been identified as a human pheromone or linked to a specific response.

Moreover, if people are giving off pheromones, scientists aren’t sure how others are detecting them. Many mammals and reptiles have what’s known as a vomeronasal organ that detects pheromones. While some human noses contain the tiny organ, it may not be functional; sensory neurons have little or no connection with the nervous system. So for now, the answer to this question remains “maybe.” And that uncertainty truly stinks.

9. What’s the Deal With Gravity?

Of the four fundamental forces of nature, gravity is the runt of the litter. It holds the universe together, but it’s weaker than its three siblings: electromagnetism, weak nuclear forces, and strong nuclear forces. How much punier is it? The next step up, weak nuclear, is 10^26 (100,000,000,000,000,000,000, 000,000) times stronger. Gravity’s relatively feeble pull makes it hard to demonstrate with small objects in the lab.

Gravity doesn’t play well with the other forces either. Try as they might, scientists can’t use quantum theory and general relativity to explain gravity on small scales. And this incompatibility leaves us short of physicists’ grandest goal: a unified theory of everything.

Worse still, scientists can’t even figure out what gravity is made of. The other fundamental forces are all associated with particles that help carry them, but no one’s been able to detect the gravitational particle—the hypothetical graviton—even with the most super of supercolliders! And while some scientists are frustrated by its elusive nature, others know it’s just gravity’s way—the force has a reputation for bringing us down.

10. How Many Species Are There?

Taxonomists have been finding, naming, and describing species in an organized manner for more than 200 years, and they’re probably nowhere close to being finished. It’s not that they’re slacking off on the job either. In the last decade alone, scientists have reported more than 16,000 new species per year; in total, they’ve cataloged 1.2 million. It’s anybody’s guess how many are left undiscovered, though. Going out and finding every single species would take the 300,000 working taxonomists a lifetime, so they have to make educated guesses.

Making these kinds of extrapolations presents serious logistical hurdles. Biodiversity hotspots often fall in developing countries, which suffer from a shortage of taxonomists. Furthermore, up to 80 percent of the planet’s life may be hiding out in hard-to-reach places under the sea.

Given these troubles, it’s no wonder there’s a wide variance in expert guesses of how many species are left undiscovered. The most recent ballpark figures place the number between five and 15 million species, which makes the odds of someone discovering a unicorn just slightly better than we’d even dared to dream.

This story originally appeared in mental_floss magazine.