When a child sees something luminous, he thinks it is performing magic. For adults, this luminous product is not only appreciated as magic, more people will look for the chemical effect behind it.
Your average glow-in-the-dark doodad gets its glow from a phosphor, a member of a group of substances that radiate visible light after being energized. Some phosphors are natural, like ones found in your teeth and fingernails, and chemists have also created hundreds of others. The ones most useful for glow-in-the-dark items are those that can be energized by normal light and have a pretty long persistence (glow time).
Take a glow powder that fits the bill, mix it in with the plastic to be molded into the product, and you have yourself a glow-in-the-dark whatever. Light from the sun or the living room lamp energizes the phosphors in the plastic and excites them, and with the lights off, you can watch as their atoms slowly lose this extra energy in the form of a dim glow.
Beyond the usual glow-in-the-dark artifact, there are some special cases where glowing products work a little differently. Glow sticks work by chemiluminescence — that is, the light is emitted as a product of a chemical reaction. Items that need to glow continuously with little or no “charge,” like clock or watch hands that glow for hours after a light has been turned off, work by radioluminescence. Timepieces like this still use phosphors to create the glow, but also have a little bit of a radioactive element like radium added to the glowing parts, which gives off small amounts of energy — not enough to be dangerous to the user, but, historically, a problem for the people who make the products — that constantly charge the phosphors in the same way a light would and keep the item glowing through the night.