A Joke

Okay, listen up, because this is gonna be a tough one for a dummy like you. This joke is relying on you knowing that knock-knock jokes end with someone saying their name when asked "Who's there?" In this case, the joke is that "A joke" is at the door, and after you ask who's there, the joke teller says "U," implying the punchline is the phrase "A joke, you." Pretty stupid, right? That's why you thought it was funny, dingus.