How old are you Mummy?

A Mum is driving a little girl to her friends house for a play date.
“Mummy,” the little girl asks, “how old are you?”
“Darlin', you are not supposed to ask a lady her age,” the Mum warns, "It is not polite.”
“OK,” the little girl says, “How much do you weigh?”
“Now really,” the mother says, “these are personal questions and are really none of your business.”
Undaunted, the little girl asks, “Why did you and Daddy get a divorce?”
“Those are enough questions, honestly!” The exasperated mother walks away as the two friends begin to play.
“My Mummy wouldn’t tell me anything,” the little girl says to her friend.
“Well,” said the friend, “all you need to do is look at her drivers license. It’s like a report card; it has everything on it.”
Later that evening the little girl says to her mother, “I know how old you are, you are 32.”
The mother is surprised and asks, “How did you find that out?”
“I also know that you weigh 140 pounds.”
The mother is past surprise and shock now. “How in heavens name did you find that out?”
And,” the little girl says triumphantly, “I know why you and Daddy got a divorce.”
“Oh really?” the mother asks. “Why?”
“Because you got an F in sex.”