Who is the mysterious person i...

Little Jenny walked into the kitchen one day and looked up at her mother, who was busy cooking dinner. "Mommy, how old are you?" she asked.
"Now dear," said her mother, "You should never ask a woman what her age is."
"Why not?" demanded Jenny.
"Because it isn't polite. You'll understand better when you grow up."
Jenny thought about it for a moment, then piped up, "Mommy, how much do you weigh?"
"Jenny," said her mother, "That's not a question you ask people."
"Why not?" demanded Jenny.
"Because it's not polite to ask grown-ups about how much they weigh. You'll understand some day."
"Mommy," Jenny asked, "Why did you and Daddy get divorced?"
"Darling," her mother replied with a sigh, "That's something that's still very painful for Mommy, and I really can't talk about it now. I'll explain when you are a little older."
The next day, Jenny told a friend at school about the conversation with her mother. The other little girl explained to her, "All you have to do is get a look at your mom's driver's license. It has all the information about any grown-up you want on it."
So little Jenny sneaked a peek in her mother's purse when she got home, and looked over her license, examining it carefully. That evening, she went back into the kitchen and announced, "I know how old you are, Mommy, You are 36!"
Her mother looked down at her, surprised. "And I know how much you weigh!" said Jenny. "You weigh 135 pounds." "Jenny, where did you learn this?", her mother asked. Jenny just smiled and continued, "And, I know why you and Daddy got a divorce." Her mother just gasped and asked, "Why?"
Jenny replied, "Because you got an F in sex!"