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? Dough.

Dough who? Dough-n't worry, I'm just here for the bread!

Knock knock. Who's there? Nana.

Nana who? Nana your business!

Knock knock. Who's there? Citation needed.

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