Knock knock. Who's there? Cows go

Cows go who? No, cows go 'moo'

Knock knock. Who's there? Justin.

Justin who? Justin time for some laughs!

Knock knock. Who's there? Egg

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

Knock knock. Who's there? The joke.

The joke who? The joke who was supposed to be original and unexpected, but now feels like it's just pointing out its own existence.