Well, she's not just spinning the stool. The part that's touching the floor is also being lifted up in a circular motion-the friction from it coming down is what's moving it around.
We used to do that at school, we had to stop after one of the special ed kids tried it and crashed into the projector. They took our stools away and gave us those ****** plastic chairs. The tard had to get stitches in his face and we all got after school detention for being 'bad influences'.
How did she even discover that she can do that? Like how many times a day are you sitting on an upside down stool wiggling your butt?
There's a story here.