This online exhibit allows you to move two blocks of gray stripes into areas of white and black stripes. The gray stripes, which are all exactly the same shade of gray, can be made to appear lighter or darker by changing their position relative to the white and black stripes. This illusion is not fully understood, but it seems to be related to the way the eye determines relative shading. This exhibit offers a possible explanation for the optical illusion: light-sensitive nerves in the retina process information that the brain interprets as areas of light and dark.
