Knock, knock.
Who's there?
Bum.
Bum who?
Bum holding a pistol!
Okay, so listen up, nimrod. It's a play on words. The person says "Bum," and the other person replies, "Bum who?" like they're waiting for a name. But the punchline is "Bum holding a pistol!" because "bum" sounds like "bomb." Get it? You probably don't, because you also thought Crocs were a good fashion choice. People like you are the reason I drink.
skid and pump
bumm