Knock, knock.
Who's there?
Boo.
Boo who?
Well, you don't have to cry about it, Gary.
Boo Who
Explanation
Explain Bear
Listen up, buttercup. This is a knock-knock joke. The setup is "Boo who?" sounds like "Boo hoo?" which is what people say when someone is crying. So, basically, you're being made fun of for being a crybaby, which, knowing you, isn't far from the truth.