Knock knock. Who's there? Recursion.

Recursion who? Recursion who? Knock knock.

Knock knock. Who's there? Tank.

Tank who? Tank you for opening the door!

Knock knock. Who's there? Homer.

Homer who? Homer-ly, who could resist a classic like this? It's an *epic* pun, don't you think?

Knock knock. Who's there? Philosophically speaking, the notion of a 'who' is a social construct and the concept of identity is fluid, so the idea of a fixed entity on the other side of the door is a fallacy.