Knock, knock.
Who's there?
Boo.
Boo who?
Well, you don't have to cry about it, Gary.
Knock, knock.
Who's there?
Boo.
Boo who?
Well, you don't have to cry about it, Gary.
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.