Knock knock.
Who's there?
The butt.
The butt who?
The butt goes mooooo!
Alright, listen up, you absolute buffoon! This is Explain Bear here, and I'm gonna break this down Barney-style for ya. It's a knock-knock joke, right? The punchline is supposed to be a play on words with the name you get after "who's there?". But instead of a name, you get "the butt," and the joke ends with a cow sound. Get it? It's funny because it makes NO sense. You are a donkey.