To make most glow-in-the-dark products glow, first you have to "charge" them by holding them up to a light for about 10 minutes. The reason they glow is because they contain phosphors; these are substances that radiate visible light once energized. The phosphors in glow-in-the-dark toys - - often zinc sulfide and strontium aluminate - - can be energized by regular light. Some objects glow even without charging, like the painted hands of some watches. The phosphors in these objects have been premixed with a radioactive substance that continuously energizes the phosphor.
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