Knock knock. Who's there? That's what I'm asking.

That's what I'm asking who? That's what I'm asking who wants to know who's on the other side of this door!

Knock knock. Who's there? Grape.

Grape who? Grape to meet you, I've heard a lot about you!

Knock knock. Who's there? Citation needed.

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

Knock knock. Who's there? Alpaca.

Alpaca who? Alpaca the suitcase if you open the door!