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 dooorway.
Timmy is shocked and runs out of the room.
The father goes, "I'll go talk to Timmy".
He goes to Timmys 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?"