grandson goes up to grandma and says ..
"Grandma, what's that thing called when one person lays on top of the other person in bed?"
Grandma, ponders the age of her grandson, but finally decides that kids these days know everything so replies "it's called intercourse, and that's how you came about"
So the grandson happily runs off.
Later in the day, the grandson comes back and says "Grandma, Sam's mother says it's a bunk bed and she'd like a word with you"