A little kid walking past his parents' bedroom sees his parents having sex. The father yells, "Shut the door!"
The kid slams the door and runs downstairs.
Ten minutes later, when the father's finished he goes downstairs and he hears a noise in the kitchen.
He opens the door and there's the kid banging the grandmother on the kitchen table. The father yells, "What the hell's going on here?"
The looks up and says, "It's not so funny when it's your mother, is it?"