Fine-tuned humor

Claire: Mom, tell me a knock-knock joke.

Me: Ooh. Okay. I've got a good one. Knock, knock.

Claire: Who's there?

Me: Boo.

Claire: Boo who?

Me: Well you don't have to cry about it.

Claire: [stares blankly]

Me: [excited] Get it? Boo hoo! It sounds like you're crying.

Claire: [still staring.]

Me: [deflated] Fine. Here's another one. Knock, knock.

Claire: Who's there?

Me: Truck.

Claire: Truck who?

Me: Truck driving down the road.

Claire: [slapping knee] Ah ha ha!! Now THAT'S a JOKE!