This animation was on my laptop for quite some time. I had it for almost a year. Today anonymous user submitted me this gif version, and when I opened it, only then I realized how great this illusion actually is. If you look at the spinning girl’s silhouette below, you will think it is spinning clockwise, probably. When you check her shadow below, momentarily the spinning direction changes in your mind, and now the girl is spinning counter-clockwise. It can be quite hard at the beginning to notice switch of the spinning direction, but eventually you’ll manage. I posted more simple illusion of this kind while ago, and you can see it here. Now, be honest - how long did it take you to see it spin in both directions? Not as easy as the hidden tiger, ayeh? I lost the link to the original artist, but best I could come up with is the place I found it on.