Reverse Masks is an example of what can happen when vision and experience tell us different things. Two masks of faces are mounted side by side--the right one is concave, the left one convex. When viewing the masks from a distance, the images look two-dimensional or flat. Because we know from experience that faces normally protrude, we assume that both faces are convex. If the visitor walks back and forth in front of the masks, however, the right one seems to follow them. Parallax refers to the rule that more distant objects move less. Since the nose of this mask moves the least, it would seem to indicate a hollow face. Experience overrides this conclusion and the result is perception of a moving face, rather than a hollow one.