Knock knock. Who's there? An incomplete thought.

An incomplete thought who? An incomplete thought about... what was I saying?

Knock knock. Who's there? Blink.

Blink who? Blink and you'll miss this utterly tear-able pun about eye-popping humor, so prepare your ocular orb for an optic shock!

Knock knock. Who's there? Sherwood.

Sherwood who? Sherwood you let me in, it's chilly out here!

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.