Note that since the cylinder is clear (we are seeing both sides), it is ambiguous which way the cylinder is spinning. As you stare at it, the cylinder may seem to suddenly change in the direction that it is spinning. This phenomenon is commonly made use of in illusions. When there are two competing interpretations of an ambiguous stimulus, the brain will see one for awhile, and then the other. The most famous of such images is the Nekker cube shown here on the left. The cube can be seen with its front most face as pointing down to the right, or up to the left. What is interesting about the cylinder, is that there are actually a huge number of possible interpretations. Each vertical segment (down to a single pixel) could be spinning in a direction independent of its neighboring segments. However, to our knowledge, everyone always sees the entire cylinder spinning in one of the two possible directions.
