The Ebbinghaus illusion, pictured above, has tricked the minds of many people who've eyed it. Most viewers see the red dot on the left as smaller than the one on the right. But if you measured them, you'd find they are exactly the same size. That's because of the way the dots are arranged. Your brain sees the left red dot as closer, given the giant dots around it, and the right red dot as farther away, given the tiny black dots around it. When the brain sees them both at once, the only way for the right red dot to be farther away than the left red dot, is if it's larger. Still confused? Here's a more in-depth description.