Knock knock. Who's there? Cows go.

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

Knock knock. Who's there? Elijah.

Elijah who? A light bulb just went off, I've got a brilliant idea for a joke!

Knock knock. Who's there? The sound of one hand clapping.

The sound of one hand clapping who? Exactly! You're not supposed to be able to answer that.

Knock knock. Who's there? Brush

Brush who? Brush-ing aside the formalities, can I come in?