Knock knock. Who's there? No one.

No one who? Exactly.

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.

Knock knock. Who's there? Fig

Fig who? Fig-ure it out yourself, I'm busy!

Knock knock. Who's there? Recursion.

Recursion who? Recursion who? Knock knock.