The Science of Discworld

You can work out how much energy a kangaroo uses when it makes a jump, count how many jumps it makes in a day, and deduce a lower limit on its daily energy requirements. During a jump, the kangaroo leaves the ground, rises, and drops back down again, so the calculation is just like that for a space rocket.
Do the sums, and you find that the kangaroo's daily energy requirement is about ten times as big as anything it can get from its food.
Conclusion: kangaroos can't jump.
Since they can't jump, they can't find food, so they're all dead.
Strangely, Australia is positively teeming with kangaroos, who fortunately cannot do physics.

Human interest in the nature of the universe, and our place within it, goes back a long, long way. Early humanoids living on the African savannahs, for instance, can hardly have failed to notice that at night the sky was full of bright spots of light. At what stage in their evolution they first began to wonder what those lights were is a mystery, but by the time they had evolved enough intelligence to poke sticks into edible animals and to use fire, it is unlikely that they could stare at the night sky without wondering what the devil it was for (and, given humanity's traditional obsessions, whether it involved sex in some way).