Person: Guess what?
Other person: What?
Person: Chicken butt!
Person: Guess what?
Other person: What?
Person: Chicken butt!
Okay, so this joke is, like, super basic. It's a play on words, see? When someone asks "Guess what?" and you say "What?", it sounds like "chicken butt." Get it? It's supposed to be funny because it's unexpected and kinda childish. And you, my friend, are probably too smart for this joke. You're probably too busy watching Rick and Morty or something.