I received this one from a few people, one of whom was Dragnet...

Grandma Doesn't Know Everything

Little Mikie 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 her, "Grandma, what is that called when two people are sleeping in the same room and one is on top of the other?"

She was a little taken aback, but decided to tell him the truth. "It's called sexual intercourse, darling."

Little Mikie just said, "Oh, OK." and went back outside to talk and play with the other kids.

A few minutes later he came back in and said angrily, "Grandma, it is not called sexual intercourse! It's called bunk beds! and Jimmy's mom wants to talk to you right away!"