I'm always on the look out for cool optical illusions. I was forwarded a link to article which features a spinning silhouette of a dancer.
The article claims that the figure indicates whether you are right or left brain dominant depending on whether you see her spinning clockwise or counter-clockwise. I'm skeptical of the scientific accuracy of that but the illusion is, never the less, very interesting.
I initially saw her spinning clockwise and had a very hard time convincing my brain to perceive her spinning the other way. I found that once my mind saw it as a three dimensional figure, it was very hard to do the usual sort of left/right flip that is typical of these illusions. In my case, the trick is to concentrate on the shadow of her foot which is my easier to flip around.
Thanks to Sarah Fry for the article link.