What is a knock joke?

A knock joke is a type of joke that is based on a pun or wordplay involving the word "knock". The joke typically begins with the question "Knock knock," to which the listener replies "Who's there?" The person telling the joke then responds with the punchline, which usually involves a play on words or a funny twist. For example:

Knock knock.

Who's there?

Dwayne.

Dwayne who?

Dwayne the bathtub, I'm dwowning!