mother's secrets

A mother is driving her little girl age 10 to her friend’s house for a play date.
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“Mommy,” the little girl asks, “how old are you?”
“Honey, you are not supposed to ask a lady her age.” 
“OK,” the little girl says, “How much do you weigh?”
“Those are personal questions and never asked.”
Undaunted, the little girl asks, “Why did you and Daddy get a divorce?”
“That’s enough, young lady!”

The exasperated mother walks away as the two friends begin to play.

“My mom won't tell me anything about herself,” the little girl says to her friend. 
“Well,” says the friend, “all you need to do is look at her driver’s license. It’s like a report card and has everything on it.”

Later that night the little girl says to her mother, “I know how old you are, 32.”
Surprised, the mother asks, “How did you find out?”
“I also know that you weigh 130 pounds.”
The mother is more than surprised. She is shocked.
“How in Heaven’s name did you find out?”
And the little girl says triumphantly, “I know why you and daddy got a divorce.” 
“Oh really? Why?”
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“Because you got an F in sex.”