The hidden room may be an identical mirror-image of the main room, so that its reflected image exactly matches the layout of the main room; this approach is useful in making objects seem to appear or disappear. This illusion can also be used to make an object, or person -- reflected in, say, a mirror -- appear to morph into another (or vice versa). This is the principle behind the Girl-to-Gorilla trick found in old carnival sideshows. Another variation: the hidden room may itself be painted black, with only light-colored objects in it. In this case, when light is cast on the room, only the light objects strongly reflect that light, and therefore appear as ghostly, translucent images on the (invisible) pane of glass in the room visible to the audience. This can be used to make objects appear to float in space.