Knock knock. Who's there? Cows go.

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

Knock knock. Who's there? Hearse.

Hearse who? Hearse a real groaner for ya: I used to be a baker, but I couldn't make enough dough to roll with the big timers!

Knock knock. Who's there? The premise.

The premise who? The premise for a knock-knock joke needs two people, and I'm talking to myself.

Knock knock. Who's there? Flake.

Flake who? Flake out, I'm tired of waiting!