A driver licences tells you all that you need to know

An inquisitive young boy is talking to his mother.

"Mommy, how old are you?"

"Oh, that's quite personal, son, and besides it's not polite to 
ask a woman her age."

"Ok, mommy, then tell me, um, how much do you weigh?"

"Well, I can't tell you that either, a woman's weight is a secret."

The boy was getting a bit frustrated by now.  "If you can't tell 
me that, can you tell me why you and Daddy split up?"

"Someday I'll explain it to you but it's all quite complicated 
and I don't think you'd understand."

The boy wandered off unfulfilled but kept all of this in the back 
of his mind.  The next day in school, he was talking to one of his 
school chums and was told to look at his mom's driver's license.  
That would have all of his answers.

So very early the next morning he snuck into her wallet and looked 
at her driver's license.  When she awoke, he said, "I know how old 
you are!"

She said, "You do?  How old am I?"

"You're 31.  And I know how much you weigh, too!"

"Oh," she said, getting curious, "how much is that?"

"You weigh 126.  And I even know why Daddy left!"

She was really curious now. "Why was that, son?"

"It's because you got an F in sex!"