Where did he learn THAT?

I know I certainly didn't teach it to him.  Seriously, I have no idea where he got it, and he nearly made me cry.

On any given day in our house, you can generally hear the following snippet of conversation, or something like it:

Benjamin : "Penelope, can I have Lightning McQueen?"
Penelope : "No, is Mine!"
Benjamin : "No, it's MINE!"
Parent : "Ben, you were over here playing with these cards.  Let her play with the car, and then you can have a turn when she's done."
Benjamin : "But she has it and I want it!"
Penelope (losing interest in the car, dropping it on the ground) : "I play on the train tracks!"
Benjamin : "Pe-Ne-Lo-Peeee! *I* was going to play with the trains!"
Penelope : "MINE!"

That happens two, maybe three (hundred) times each day.  After a minute or two, they both settle down and start doing their own thing for a while, sometimes cooperating and playing together until the next big argument.   But just before bed tonight, something happened that I'd never witnessed before.

Mommy : Ok Penelope, go get the book you want for bed."
Benjamin : "Penelope, do you want one of my Magic Tree House books for bed?"
(note, these books are totally his, they have far too few pictures to keep here interested, and they are all kept in his room, generally off-limits to her for fear of the inevitable kid-plosion that would occur if she ever so much as glanced at one)
Penelope : "Mmmm... Yeah."
Benjamin (leading Penelope to his bookshelf) : "Ok, which one do you want? You can pick one."
Penelope : "Mmmmm..."
Benjamin (picking one for her)  "How about this one? Its a really good one."
Penelope : "Ok."
Benjamin : "But you have to give it back tomorrow, ok?"
Penelope (carrying the book into her room) : "Ok."

Holy crap!  Not only did he just share one of his most-prized possessions with his sister, it was his idea, and he lent it to her overnight!  Hopefully she's willing to give it back in the morning, and it is still in one piece.