Little Johnny was staying with his granny for a few days.
He'd been playing outside with the other kids for a while when he came into the house and asked her, "Granny, what's that called when 2 people sleep in the same room and one is on top of the other?"
She was a little taken, but she decided to just tell him the truth. "It's called sexual intercourse, darling".
Little Johnny just said, "Oh, OK," and went back outside to play with the other kids.
A few minutes later he came back in and said angrily, "Granny, it isn't called sexual intercourse.
It's called "Bunk Beds". And Jimmy's mom wants to talk to you."