Tires
What to see and do: If you stare at the dot in the middle of the "spiral," the two circles will appear to be rotating in opposite directions. In fact, they are not rotating at all. They are only growing and shrinking.
What's happening: The bends in the circles cause the illusion. The exact reason this illusion works is not easy to explain. See "Trick Eyes" by Akiyoshi Kitaoka for many more of these amazing illusions.
Credit: This illusion is based on one in the book "Trick Eyes" by Akiyoshi Kitaoka.