Grammar Devil

I don't know why I came up with this today, it kind of just happened. For what it's worth, I think it's pretty clever.

I Apologize in Advance

Knock, knock, knock Doctor . . . (x 3)

This morning as I was waking from sleep, I don't know why, but I had this thought:

Knock knock.

Who's there?

Yes, the Doctor. 

Doctor Who?

Precisely.