What do you see in this image? It appears to be a mishmash of overlapping or intertwining circles, or maybe even a spiral of some kind. But when you try to follow those circles individually, you'll find that there are four concentric circles that do not overlap at all!
This is called Pinna’s Intertwining Illusion. The reason you see intertwining circles, or a spiral of some sort, is due to the orientation of the squares, which varies from circle to circle. The squares are uniformly close together, but because they are squares, they appear more angular in the smallest circle. The overall effect makes our brains see overlapping circles. It's like if you took a Twist-O-Flex watchband and tried to roll it up tight until the links separate enough to pinch you, which is the only example I could find within reach at my desk. In other words, we are squaring the circle.
This illusion was discovered by by psychology professor Baingio Pinna. He has conducted research into the brain's processing of illusions displaying the rotating motion effect. That sounds like illusions that will make you woozy, and they do. -via TYWKIWDBI
(Image credit: Jochen Burghardt)