Ask grandma!

Little 7 year old Johnny was staying with his grandmother for a few days.
He'd been playing outside with the other kids for a while when he came into the house and asked grandma, "what's that called when two people sleep in the same room and on is on top of the other?"

Granny was taken off guard. No question like this had come up for years, but she decided to tell him the truth.
"It's called sex, darling"

Little Johnny said, "oh, OK" and went back outside to play with the other kids.

A few minutes later he came back in and said as mad as he could, "Gramma, it isn't called sex. It's called bunk beds. And Jimmy's mom wants to talk to you."