Knock knock. Who's there? A philosophical question.

A philosophical question who? If a joke falls flat in the forest, and no one laughs, was it ever funny?

Knock knock. Who's there? No one.

No one who? Exactly. There's literally nobody here. I just wanted to see if the door was locked.

Knock knock. Who's there? I'm not sure.

I'm not sure who? I'm not sure who is asking, can you please state your full name and the purpose of your visit?

Knock knock. Who's there? Wanda.

Wanda who? Wanda why you won't open the door!