This is a psychology-of-perception demonstration you can experience in a room whose walls are all painted the same color. Look into a corner of the room, where two walls and the ceiling meet. Almost certainly one wall will be receiving more light than the other from some source, so one wall will look lighter than the other. Yet, somehow, your visual perceptual system correctly tells you that you are seeing two walls, painted the same color, meeting at a corner. Now close one eye and hold up pieces of cardboard so as to block out the ceiling and the floor, restricting your view to the two walls and the corner where they meet. The scene will then look dramatically different. You’ll see two flat-looking areas of color meeting at an edge, like two paint sample chips butted against one another. The impression of two walls meeting at a corner will be gone.