This illusion has been making the rounds lately:
Can you spot the 16 circles? It may take a minute, but if you look closely between the square panels, you may see them.
This is known as the Coffer Illusion; coffers are the sunken panels that often decorate ceilings, which is what most people see when first looking at the image above.
Of course, the image contains no panels or circles at all—just shaded lines arranged in a pattern. It’s your brain that turns this pattern into something coherent. What I find fascinating is that once you see the circles, you can switch back and forth between interpretations. Even if you were originally biased toward seeing square panels, the conscious parts of your brain can force a different interpretation, once you know what to look for.