system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it funny when people fall over?
gpt
There is no universally accepted explanation of the role of humour and how it works. One possibility, however, is that humour is one of the possible reactions to mental stress caused by contradictions.

When something is troubling, confusing, or strange, it causes some tension. This may lead to fear, anger or other emotions, but when it turns out that there is no danger and nothing to worry about, we laugh as a way of expressing relief.

Humour makes use of that by creating a situation where there is some contradiction, tension or weirdness (a setup) and then resolving it in a way that one does not see coming (a punchline, a payoff). This is why humour is usually based on either something absurd, or on someone's misery that we can distance ourselves from. Both create a situation where there is something that might worry us, but we are shown that there is no reason to *personally* worry.

People falling over can be either. It is a situation where someone is hurt, but if the situation is otherwise harmless (the fall is exaggerated or non-threatening), it is funny. Our brain knows there is no danger, so it reacts by causing us to laugh - to show that we are relaxed to other people, that there is no reason to be afraid or concerned for the person who fell. Notice that we don't usually laugh when someone falls over and dies or gets badly hurt (unless the person is presented in a way that makes us not care about them at all, for example being a villain or otherwise someone we perceive as inferior).

The other reason, contradiction and absurd, is usually by contrasting the fall with something that doesn't "fit" - for example, a very serious and important-acting person falls down, so our mental idea of the person being serious and dignified is contrasted with us seeing them in a very undignified situation. Or in another situation - a person may fall unexpectedly, so the fall in circumstances we didn't expect causes a situation that also feels contradictory. Or in yet another scenario, the fall looks so silly and out-of-place that it creates a similar feeling of being absurd.