That’s because such toys contain phosphors. A phosphor is a chemical that has the property of luminescence, that is, the emission of light by a substance that has not been heated. Phosphors radiate light after they are energized. Light from the sun or the lamp in the room energies the phosphors and excited them. When the light is switched off, the atoms slowly release the extra energy and give us the dim glow. So, the glow-in-the-dark toys need to be exposed to light, or charged. Zinc sulphide and strontium aluminate are the two most commonly used phosphors.