A little boy walks in on his parents having sex.
“Mommy, why are you on top of Daddy?” he says. The mom thinks fast and says “Well, your daddy has a big belly, so sometimes I get on top of him and try to flatten it out.” The boy says “well that will never work.” “Why?” says the mom. “Because when you go out shopping on Saturdays, the lady next door comes over and blows it right back up again.”