Trio with grandmother and grandfather for granddaughter to learn everything.

Her grandmother and grandfather had practically raised her, without a mother and father, she had learned everything about life from them. So when she grew up and walked into her grandparents' room and found them naked on top of each other, they decided they should start teaching her something more complex than math. They explained to her the need for her to grow up and marry a man, and since she had to fulfill everything he asked for, they undressed her little by little, teaching her how she should touch herself and him to reach the pleasure that they were allowed to have as spouses.