Knock knock. Who's there? This isn't a door.

This isn't a door who? Exactly, you're knocking on a wall.

Knock knock. Who's there? Ken.

Ken who? Ken I come in already? It's cold out here!

Knock knock. Who's there? Donut.

Donut who? Donut even think about leaving me out here!

Knock knock. Who's there? Citation needed.

Citation needed who? This joke requires a source to be funny.