This is a file from the Wikimedia Commons
Created by: Edward H. Adelson Source: web.mit.edu
Are squares A and B different colors or are they the same color?
Click on the image and see the result -- and if that doesn't work, click on the image at Astronomy Picture of the Day (July 17, 2007)
For discussion see the Wikipedia article
Why does the illusion work? See Explanation
But take note of the following important (and atypical) moral by Adelson: "As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view."Aha! Human beings are not idiots after all. Even their sense organs work pretty well -- indeed, amazingly well, all things considered. (This is my moralizing.)