Knock knock. Who's there? No one.

No one who? Exactly.

Knock knock. Who's there? This is a knock-knock joke.

This is a knock-knock joke who? Just trying to figure out where we go from here.

Knock knock. Who's there? Ophelia.

Ophelia who? Ophelia-ing pretty good about this joke, even if it's a bit of a tragedy for your funny bone!

Knock knock. Who's there? Egg

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