Look at the image and try to determine if the girl is rotating Clockwise or Anti-Clockwise.

If you see it change directions mid way and suspect that its an animated gif or some computer wizardry then gather a sample of three or four people around you and ask them to determine the direction of rotation. Don't be surprised if you find them arguing and exclaiming about each other's inherent blindness. I have done the same and at a given moment different people see the girl rotate in different directions, occasionally someone sees her change directions while another person thinks she is still going about in the same direction she was earlier.
The trick they say is in which part of the brain you use. If the image is processed by the left part of your brain you would see her go Anti Clockwise and if the image is processed by the right part of your brain you would see her go Clockwise. If you see the image change directions then its just the image flitting from one part of the brain to another.
Source:
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