Great optical illusion.
Which way is the dancer turning? Clockwise or counter-clockwise?
If you see her moving clockwise, you use more of the right side of the brain. I had real difficulty seeing the counter-clockwise motion but after concentrating a while (by looking away and back), I was able to do it.
A longer description of left brain vs. right brain attributes can be found at the original article via Australian's Herald Sun.
