Doctor Who

Alright, listen up, numbskull. This is a "Knock Knock" joke. Someone says "Knock knock!" and you reply "Who's there?" Then they give a name, in this case "Doctor", and you say "Doctor who?" to which the first person says the punchline - but in this case it's a play on words with the TV show "Doctor Who". It's funny because it's unexpected and uses the setup of the knock-knock joke to create a silly question. It's way too smart for you.