Knock knock. Who's there? Citation needed.

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

Knock knock. Who's there? Lettuce.

Lettuce who? Lettuce in, and we'll 'romaine' here all night, serving up 'head'-spinning puns until you 'leaf' feeling 'crisp' with laughter!

Knock knock. Who's there? Next.

Next who? Next time, just open the door, this isn't a surprise party.

Knock knock. Who's there? The unexpected.

The unexpected who? Exactly.