A guy's having sex with his wife. All of a sudden he looks over, and there in the doorway is his 8 year old son.
The kid looks horrified and runs away.
The guy says to his wife, "Well, I'd better go talk to Timmy".
He puts on his clothes, opens up Timmy's bedroom door, and there's Timmy nailing his grandmother!
The father says, "OH GOD!"
The kid says, "Not so funny when it's your mom, is it?"