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.

Knock knock. Who's there? Cows go.

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

Knock knock. Who's there? Peas.

Peas who? Peas-fully waiting for you to open the door!

Knock knock. Who's there? Egg

Egg who? Egg-cuse me, I'm cracking under the pressure to make a good pun.