Knock knock. Who's there? Sew

Sew who? Sew long, I've been trying to weave a good joke.

Knock knock. Who's there? Egg

Egg who? Egg-cuse me, I'm cracking under the pressure to make a good pun.

Knock knock. Who's there? Dictionary.

Dictionary who? I'm not defining the terms of this conversation.

Knock knock. Who's there? Cows go.

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