There is the ISIS, called the most difficult puzzle ever. It sure had put many minds and brains to test, but here is something that'll put the two halves of your brain confronting head-on for supremacy? Asking your witty bit to decide which half of it was more dominating, helping you figure out things, and did you always figure them out correctly. It is the test of the Right brain with the Left brain, and it's time to concentrate, not on the lady, but her movement, that's gonna decide the rest. If you see her moving clockwise, then you're using the creative side of your brain, that is the Right. Whereas, if you see her move anti-clockwise, then you're using the logical part of your brain, that is the Left.
The right and left side of the brain control two different modes of thinking, with each of us preferring one mode over the other. The majority of people in this world are left brained but some equally adept at both modes and are whole brained. The dancer changes direction due to lack of dimension of the shadowed two dimension girl, with its solid black image without interior detail. This causes your brain to not know which leg is in front of the other.
My first impression of the dancer was that it is moving anti-clockwise, and that's how most of you would see it too. However, if you try and focus, who knows you may see it move clockwise, if you do, please let us know, by leaving your experiences in the comment section.