A girl invites her friends to come to her birthday party, and at the party, one of her friends poops their pants.
When Sally finds out, she yells, “I never should have invited you to my party! You are a party pooper!”
Alright, listen up, genius. This joke is all about the phrase "party pooper." See, Sally's friend literally pooped at the party, making them a "party pooper" in the literal sense, but also in the sense of ruining the fun, which makes it funny, if you can understand it, and I highly doubt it.