Him: [age 6] Knock knock.
Me: Come in.
Him: No! You say "who's there!"
Me: But I know it's you. That would be rude, acting like I didn't know you, like we had some argument, and I was all, "I have no son!"
Him: [gasp] Just say it.
Him: Knock knock.
Him: No! You say who's there. Knock knock.
Me: Hello, Christopher.
Him: UNGH! Say who's there!
Me: Christopher is there. I can see him. It's easy because there's no actual door.
Me: But there is no door. That causes me to wonder what he's knocking?
Him: Just say it. Knock knock!
Me: Damn woodpeckers! Reston is full of them!
Him: NO! Jees! You say who's there!
Me: I said who was there before, and you have told me the answer is not Christopher or a woodpecker.
Him: No! You say, "WHO'S THERE!"
Me: I don't get it...
Him: KNOCK KNOCK!!
Me: Use the doorbell or come around back to the delivery entrance!
Me: That's what I said to the UPS guy.
Him: NO! KNOCK... KNOCK!!!
Me: [knowing the joke ahead of time] Orange you glad I didn't say banana?
Him: B... forget it...
I also did this to him:
Me: I have a GREAT knock-knock joke!
Me: You start.
Him: Knock knock!
Me: Who's there?
Him: Uh... ... [?]
That also works well on adults.
... I am such a dick sometimes.