The top and bottom planes of this shape seem to be distinct, separated by a thin band of shadow. In reality, both are the same absolute color.

Your brain perceives the gradients where the tiles meet as curves, and you surmise that light hits the large, flat surface of the top tile and fades to shadow at its curved bottom edge; for the bottom tile, the opposite holds. Automatically your mind fits the drawing to a physical object that makes sense based on your experience, altering your visual perception to match expectations.