Stealing this question from the You Made It Weird podcast. It can be anything and don't worry about it not making sense due to context. Also don't worry about it being mean because often those are the funniest things since we feel bad about laughing.
I think mine might be this thing that happened in high school. I was in gym class and, since this is Texas and the gym teachers are also football coaches who are planning for the football season, there was nothing to do. So I'm sitting there with my two friends in total, bored silence. Out of nowhere, one of them picks up a hair on the other's gym shorts and flicks it away. He says "Jacob has hair on pants."
To this day, I cannot tell you why this was so fucking funny but there you go.