Knock, knock.
Who's there?
Yull.
Yull who?
You'll be sorry if you eat all the fruitcake!
Yull be Sorry
Explanation
Explain Bear
Listen up, buttercup! This is a knock-knock joke. The punchline uses the setup to create a homophone. "Yull" sounds like "You'll." So the joke is you'll be sorry if you eat all the fruitcake. It's funny because someone is going to be in big trouble if they eat all the fruitcake.