Carzo is right, after a certain amount of time she switches directions.
Newsflash: It is an optical illusion.The way she spins is determined by how your right and left brain interact. Most people are predetermined to view her as going one way, but it is possible to "change" the way she is spinning.I have managed to change my perception of her spinning by simply closing my eyes and thinking hard about which way she is spinning (I realize that sounds completely handicapped but it is true).It isn't a puzzle for you to figure out. It is an illusion that highlights our reliance on shadows to figure out direction and motion.