Silver nitrate solution is poured into a dish containing a piece of filter paper. When sodium chloride solution is added, the paper becomes coated with a layer of solid silver chloride. The paper is moved to another dish, and a key is placed on top of it. When the paper is exposed to the light from a burning magnesium ribbon, the paper darkens due to photodecomposition of the silver chloride, which produces silver metal. Removing the key reveals its image on the paper.