Little Bobby is passing his parents' bedroom in the middle of the night, in search of a glass of water. Hearing a lot of moaning and thumping, he peeks in and catches his folks in "the act."
Before dad can even react, Little Bobby exclaims "Oh, boy! Horsie ride! Daddy, can I ride on your back?"
Dad, relieved Bobby's not asking more uncomfortable questions, and seeing the opportunity not to break his stride, agrees.
Bobby hops on and daddy starts going to town. Pretty soon mommy starts moaning and gasping. Bobby cries out "Hang on tight, Daddy! This is the part where me and the milkman usually get bucked off!"