Tire Swing
Tire Swing
[Science Girl and a girl with long curly hair, are standing under a large leafless tree as Science Girl adjusts a tire swing hanging from the largest of the branches of the tree. The tire hangs so high that the small girls only reach up to just above the center of the tire which has a diameter of more than half the height of the girls.] Science Girl: OK, looks good.
[Zoom in on the girls so only the tire swing can be seen, and nothing of the tree. They both look at the tire.] Science Girl: I read that there are these huge dumps everywhere full of millions of old tires that no one knows what to do with.
[Same setting but Science Girl looks up at the tree (outside the frame).] Science Girl: We should use one of those next time. Curly haired girl: Yeah. That guy was real mad. Science Girl: I would not want to fight him again.

In this comic, Science Girl and another girl have just completed a tire swing , a common makeshift swing is created by hanging a car tire from a length of rope, typically tied to the branch of a tree as in the comic. The other girl might at first look like she has hair like Megan but not quite as she is revealed upon zoom in to have curly hair. That they are rather small kids can be seen from the size of the tire compared to them. (They could be the same as the girls in the last panel of 1580: Travel Ghost ).
In the second panel of this comic Science Girl muses that there are huge tire dumps filled with nothing but old tires that have no use. In the last panel, Science Girl continues that maybe they should use a tire from such a dump next time they make a tire swing. The presumption is that perhaps they used a brand new tire, or a tire from some other source. This is confirmed by the other girl's response (and also by the title text, see below) which makes it clear that the tire they used was in fact stolen from a guy's vehicle. The last reply from Science Girl suggests the victim put up a fight and they had to take the tire by force. So these two small girls actually fought an adult man over his truck and won the fight.
Vehicle tires have a limited lifespan. The natural end of their life is when the pattern of raised treads on the circumference of the tire, which promote traction on wet roads, are worn down to a point where they are no longer effective enough, or after 6-10 years (sunlight causes the rubber to degrade, so the tire becomes prone to cracking and unsafe, even if it appears to be in good condition). Tires can also become damaged in other ways, such as puncture.
Used tires are a notable ecological problem for a number of reasons (e.g. their size, the quantity produced, their relatively short lifespan, and the fact that they are difficult and slow to break down and contain a number of components that are ecologically problematic). A tire swing represents a functional use for otherwise useless old tires. The amount of tires (it is estimated that 259 million tires are discarded annually) makes them attractive targets for recycling. More than half of used tires are ultimately simply burned for their fuel value (which prevents them from sitting in landfills indefinitely, but this may even be worse as it releases otherwise locked up carbon thus releasing this into the atmosphere and making global warming even worse). Some steel mills that use electric arc furnaces will mix shredded tires with their scrap when charging the furnace for both the carbon value and fuel value, in place of the coal that would otherwise be used.
The comic is thus clearly Randall's attempt to draw attention to this huge ecological problem, as he so often before has done with other climate change/global warming related comics. (Climate change, especially global warming, is a recurring theme in xkcd). So while this is not the joke of the comic, it could be the point of it.
He also suggests another way to use old tires. It should be noted that used tires are not necessarily safe to use as a kids' toy as they could become sharp/frayed along the edges and stones and other hard/sharp objects may have become stuck in the tires (even going all the way through), during its life span, or worn thin enough to tear apart mid-swing (when the stresses on the swing material would be at their peak). So tires bought for use as a swing may even be made from a new tire, but not necessarily of the same solid type as those used for cars. Used tires reused for a swing should be inspected for the problems mentioned above.
The title text goes further, suggesting that they actually stole the victim's entire truck - possibly just to harvest the tire needed for the swing - and that he unsuccessfully attempted to recover the truck, so they probably did fight him. He put up enough of a fight that they do not wish to fight him again (so he at least survived). Further, since the girls expect him to try again (maybe recovering the truck with only three tires), they apparently still have the truck. One of the girls suggests that if they could find one of these tire dumps, then they could take a tire from there, make a new swing, and then just walk or run away from the truck when the guy comes back, letting him have it if he really wants it so bad.
The reason Science Girl made this swing could be that she wishes to become a cosmologist as a reference back her meeting a cosmologist on a tire swing in 1352: Cosmologist on a Tire Swing .
Note that Calvin and Hobbes , which has often been referenced in xkcd , has done the same to Calvin's father as the girls did to the guy (though without the violence) in a similar comic .
This was the first of two Wednesdays in a row where Randall used two children to make a reference to an environmental issue, the second being 1662: Jack and Jill about fracking also with Science Girl.
[Science Girl and a girl with long curly hair, are standing under a large leafless tree as Science Girl adjusts a tire swing hanging from the largest of the branches of the tree. The tire hangs so high that the small girls only reach up to just above the center of the tire which has a diameter of more than half the height of the girls.] Science Girl: OK, looks good.
[Zoom in on the girls so only the tire swing can be seen, and nothing of the tree. They both look at the tire.] Science Girl: I read that there are these huge dumps everywhere full of millions of old tires that no one knows what to do with.
[Same setting but Science Girl looks up at the tree (outside the frame).] Science Girl: We should use one of those next time. Curly haired girl: Yeah. That guy was real mad. Science Girl: I would not want to fight him again.
