Circle Or Spiral?

  

At first glance, this looks like a spiral. But use your finger and trace the line, starting in the middle or outside. You'll never get to the middle because they are actually concentric circles!

How does this work? First, the background draws your eyes to the center. But mainly it works because of how the circles are created. Instead of being a solid line, you'll see that each circle actually looks like a two shades of something twisted together, and that further leads your eyes into the center.