Knee-Slap Meets Face-Palm

(I am an elementary school teacher. I am talking to a third grader and she tells me a joke, so I say:)

Me: “Haha! That’s a knee-slapper.”

Student: “What?”

Me: “A knee-slapper. You know, when something’s funny, you slap your knee?”

(I demonstrate.)

Student: “Um, maybe that’s how they did it in the olden days, but now we just laugh when something is funny.”