What direction is this spinning dancer going? Clock wise or contrary clock wise? Well, both. Because in fact we don't know and the way you will see it is personal to you. Why? Because this image created by the Japanese Web designer Nobuyuki Kayahara, does not show any perspective, so the brain has to guess it. Depending on your own brains guess, you will see her turning to the left or to the right. You might also be able to see her turning in both (look at the standing leg and consider it once being the right leg and once the left ;))
What I love about optical illusions, it is that it proves that perception is not just a matter of eyes. It shows the impact of the brain on what we see.