Knock knock. Who's there? A placeholder for a name.

A placeholder for a name who? Exactly. It could be anyone, couldn't it?

Knock knock. Who's there? Isabelle.

Isabelle who? Isabelle not ringing, what happened to your doorbell?

Knock knock. Who's there? Lettuce.

Lettuce who? Lettuce in, and we'll 'romaine' here all night, serving up 'head'-spinning puns until you 'leaf' feeling 'crisp' with laughter!

Knock knock. Who's there? The door.

The door who? The door you just knocked on. It's not a person, it doesn't have a name.