Little Johnny 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 Johnny exclaims: "Oh, boy! Horse ride! Daddy, can I ride on your 
back?" Daddy, relieved that Johnny's not asking more uncomfortable 
questions, and seeing the opportunity not to break his stride, agrees. 
Johnny hops on and daddy starts going to town. Pretty soon mommy starts 
moaning and gasping. Johnny cries out: "Hang on tight, Daddy! This is 
the part where me and the milkman usually get bucked off!"