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What: This visual phenomenon (although I'm not completely certain it belongs to the illusion category) illustrates an effect that takes place when you watch a moving pattern for a specific period and then start looking at something else. This can happen naturally in the real - world, like when you look at a waterfall and then at the cliffs next to it. I've also programmed another version of this similar effect on this site! It uses straight lines instead of a spiral.

How To: In this example, you have to gaze at the center of the spirals. The inner part of the spirals is rotating in one direction while the outer part is spinning in the opposite direction. After about twenty to thirty seconds, you can look away. It's best to look at some wall art, outside, or your hands, and you'll notice an opposite motion occurring.

Explain It: This is another case that boils down to how your neurons process information. It's something I don't fully understand, but I'm glad to cite relevant knowledge. Neurons that code for a particular movement decrease their responses as they are exposed to a constantly moving stimulus over time; this is called neural adaptation. One theory is that the perception of stationary objects, such as rocks beside a waterfall, is represented as the balance among the baseline responses of neurons that code for all possible directions of motion.