OMG this one's a mind-fck. When you first look at this animated image, you'll see the girl spinning in one direction or another (I saw her as spinning counter-clockwise, but I think most people will see her as spinning clockwise). If you stare at her shadow/foot for a while, you'll be able to flip it so you'll now think she's spinning the opposite way!
It took me quite a bit of staring to get her to flip the first time (focusing on the foot really helped me). And then, for a while, I couldn't go back! Now after a bit more staring I can change which direction I perceive the spinning at will.
