Don't believe everything you see. Take this insane optical illusion, which uses light and shadow to transform a gray square into a white square right before your eyes:
It's a pretty impressive effect -- but it's not entirely new. The Checker Shadow illusion was published back in 1995 by MIT professor Edward H. Adelson, though optical illusion guru 'brusspup' really gives it new life by bringing it off the page and into the real world.
So how does it work, exactly? The key lies in the shadow cast by the cylinder and how our poor, confused brains process light. The shadow darkens the central square, but it still appears to be "light" in color because of the contrast with its darker neighbors. Our brains are so fooled by this local contrast that it's difficult to perceive that the "light" square in the shadow has the same absolute shade as the "dark" square out of it.
For a more detailed explanation, check out Adelson's original take on the illusion. His brain is MUCH bigger than ours.