system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does laughter, a physiological response to humour, exist?
gpt
For the type of laughter when, for example, playing, it's a signal to the person with whom you're playing that everything is lighthearted and that they aren't hurting you. Even rats do that.

For the type after a joke, the leading theory is that before you hear the punchline, your brain is trying to come up with the answer and sending signals to all kinds of parts of your brain to find out what's coming. When the unexpected (it has to be unexpected, that's what makes a joke a joke and why they stop being funny the more times they're told) punchline hits, all that energy in your brain has to go somewhere, which results in light convulsions in the form of laughter.