Knock knock. Who's there? An anticlimax.

An anticlimax who? Exactly.

Knock knock. Who's there? Cows go.

Cows go who? No, cows go 'moo'.

Knock knock. Who's there? Nobody.

Nobody who? Exactly. Why are you talking to the door?

Knock knock. Who's there? Existential Crisis.

Existential Crisis who? I'm not really sure, because the concept of identity and self is fleeting and meaningless in an infinite universe.