If you're like most people, what you see is a spiral, a series of interlocking circles. But there's no spiral there. It's actually just four concentric circles - a small circle in the middle, three progressively larger circles as you move outward. The circles are perfectly round and do not touch. Look again - wait, it's a spiral! No, it's not. Take your cursor and trace over each circle. You'll see they don't touch.
Illusions are fun! (Not to mention being evidence of how easy it is to fool our brains.)