Knock knock. Who's there? The setup.

The setup who? The part of the knock-knock joke that comes before the punchline, obviously.

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? Oblivion.

Oblivion who? *everything ceases to exist*

Knock knock. Who's there? Noah.

Noah who? Noah body's home, I'm ark-ing around for someone to let me in.