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? Silence.

Knock knock. Who's there? Dough.

Dough who? Dough-n't worry, I knead to come in.

Knock knock. Who's there? Tank.

Tank who? Tank you for opening the door!