Knock knock. Who's there? Existential Crisis.

Existential Crisis who? I'm not sure, I've been questioning the meaning of my own existence and the nature of identity, so the concept of a 'who' is somewhat fluid to me.

Knock knock. Who's there? Existential Crisis.

Existential Crisis who? I'm not sure, but the meaninglessness of this joke is a reflection of the void that lies at the heart of our existence.

Knock knock. Who's there? Maple.

Maple who? Maple you a favor and open the door!

Knock knock. Who's there? Recursion.

Recursion who? Recursion who? Knock knock.