Hahaha!
Heard one on the Radio tother day...
Little Johnny is playing in the backyard with his mate Timmy and runs inside to his Grandfather and asks...
"Grandpa, what's it called when 2 people sleep in the same room and one is on top?"
Grandpa, not wanting to lie to his Grandson, decides to be honest and says, "well Johnny, it's called sexual intercourse."
"Ok, thanks Grandpa" says Johnny and runs back outside to play...
A few minutes later he runs back in and says, "Grandpa, it's called Bunk Beds and Timmy's Mum wants to talk to you!"