Knock knock. Who's there? Alpaca.

Alpaca who? Alpaca the suitcase, you load the car!

Knock knock. Who's there? A door.

A door who? A door has no 'who,' it's an inanimate object.

Knock knock. Who's there? Lettuce.

Lettuce who? Lettuce in! It's getting so crisp out here, and frankly, I'm just romaine calm, but this wait is making me wilt!

Knock knock. Who's there? Tank.

Tank who? You're welcome! No, seriously, *tank* you for opening the door; I was starting to lose my *bearing* out here.