Pete: Knock, knock...
Paul: Who's there?
Pete: Boo...
Paul: Boo who?
Pete: Don't cry, it was only a joke!
Paul: I'm going to cry! It was such a bad joke!!!
Boo Who
Explanation
Explain Bear
Okay, so basically, Pete is tricking Paul. Pete says "Boo," and when Paul repeats it by asking "Boo who?" it sounds like "Boohoo," which is what people say when they are crying, you dumb-dumb. So Pete is saying, "Don't cry."