All “glow-in-the-dark” products contain phosphors and have to be “charged” in order to attain a good glow.  A phosphor is a substance that radiates visible light after being energized.  After being exposed to light, most items will glow for at least 10 minutes. You may need to be in nearly complete darkness to notice it.  The brighter the light and the longer the exposure, the better and longer the item will glow.