Cafe Wall Illusion
This tiled wall appears to be crooked but it is not. The illusion is caused by the spacing of the tiles and the color of the grout—the stuff between the tiles. The illusion works with gray grout, but not white or black. It is known as the cafe wall illusion because it was first noticed on a wall in a cafe in Bristol, England.
(c) Exploratorium, www.exploratorium.edu