A man and his wife are having sex. They're going at it hot and heavy. 
Suddenly they hear a noise. It's their little son Timmy standing in the 
doorway. Timmy is shocked and runs out of the room. The father goes, "I'll 
go talk to Timmy." He goes to Timmy's room. He opens the door and little 
Timmy is giving it hot and heavy.. To grandma. The father goes, "Oh my God. 
" And little Timmy says, "not so funny when it's your mother, is it?"