little Billy comes home from kindergarten for lunch

One day, little Billy comes home from kindergarten for lunch. Not
   finding his mother in the
   kitchen, or the living room, he heads upstairs to check her bedroom.
   He opens the door,
   and what does he see, but his father, who had also come home for
   lunch, stripped naked,
   on top of his mother, also naked, heavily into the act of lovemaking.
   Not wanting to
   traumatize the boy, the parents continue as if nothing was wrong.
   Billy watches, and after a
   couple of minutes asks,"Daddy, can I climb on and have a horsie ride?"
   "Of course, Son,
   we're a family." So Billy climbs on and after a few more minutes his
   mother starts
   moaning and writhing wildly. "Hang on Dad!", cries Billy, "this is
   where me and the
   mailman usually falls off!"