coalescent for pointing out what might be one of the best optical illusions you are likely to see. The silhouette seems to be rotating clockwise, but if you concentrate you can make it reverse, and seem to be going counter-clockwise. Until you see it switch you'd think it's impossible.
I suppose the reason is that you are constructing a three-D model from a 2-D image, and either model is compatible with the image, but each is completely different. It's very strange.
The way I made the image switch was to look at the text below the picture, from which angle it's impossible to construct the 3-D model. Seen like that the swinging leg looks like a pendulum, going back and forwards not round and round, and it's possible to choose which model you want to use.