My kids have been into knock, knock jokes for a long time now. We always get the traditional one's like the banana one, the orange one, and the lettuce in one. They have become pro's at making up their own, Caleb especially.
A couple of weeks ago Afton came up with this one all by herself.
Afton: Knock, knock
Me: Who's there?
Me: Stranger who?
Afton: Why did you let me in?!?!?!
I'd say we taught her pretty well when it comes to answering the door to strangers or letting them in.