Knock knock. Who's there? Tank.

Tank who? Tank you for opening the door!

Knock knock. Who's there? Existential Crisis.

Existential Crisis who? I'm not sure, but the meaninglessness of this joke is a reflection of the void that lies at the heart of our existence.

Knock knock. Who's there? Cows go

Cows go who? No, cows go 'moo'

Knock knock. Who's there? Dew.

Dew who? Dew-n't worry, it's just a drop in.