Knock knock. Who's there? Cows go.

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

Knock knock. Who's there? Alpaca.

Alpaca who? Alpaca the suitcase, we're going on vacation!

Knock knock. Who's there? This isn't a door.

This isn't a door who? Exactly, you're knocking on a wall.

Knock knock. Who's there? Tank.

Tank who? Tank you for opening the door!