The Checker Square Illusion

In this illusion, square A looks dark and B looks light, but they are actually the same color!

If you have doubts, try this - load a copy of this picture into Photoshop, cut a piece out of square A, and drag it over to square B (or vice versa). It’s cool to see. You can also use Photoshop’s eyedropper tool to measure the color (or the Colorzilla plug-in for Firefox) and you’ll see that both squares are RGB 106,106,106.