At first glance, the green table appears longer and thinner than the red table, which looks to be more or less square. Yet the two actually have the same length and width, if the red table were rotated one quarter turn.

They seem so different because the angle at the lower left corner of the green table makes it appear more "vertical," as though its back edge is receding into the distance. In response, our brains alter the real information to fit what we expect to see: a three-dimensional scene in which perspective is paramount.