Knock knock. Who's there? Tank.

Tank who? Tank you for opening the door!

Knock knock. Who's there? Sew

Sew who? Sew long, I've been trying to stitch together a decent joke.

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'.