People often don’t believe that this spinning ballerina image changes direction because of your perception. Most people see it spinning clockwise at first, and then perhaps changing direction. For some people it ONLY spins clockwise or counter-clockwise. See the original full-size image here for better viewing. If it won’t change direction for you while you’re staring at it, try looking up, down, or to one side and then looking back. Or if you’re capable of consciously changing your thought processes, try switching between conceptual/non-verbal thoughts and linear/verbal thinking.  According to everything I’ve read, this shouldn’t affect my perception, but it does. Most of the more reputable sources I found suggest the illusion is simply a result of Multistable Perception. Which to me is like saying “I know how gravity works…it makes things fall!” The original creator of this image, Nobuyuki Kayahara, has some other interesting stuff here.