Little Johnny has a habit of peeping through his parent's door everytime he hears some noise from that quarter.
One morning he told his mother he had seen her and his father with his father on top, and asked what they were doing. The mother replied. "making a little brother for you".
The next time the mother was on top, and when Johnny asked, the mother said "Making a little sister for you".
The next time when Johnny went in to peep at his parent's door, they were doing it doggy style, Johnny threw himself inside the room and screamed "Stop! I don't want a dog!"