Glowing in the dark also known as luminescence simply requires chemicals that store energy when exposed to light. These special substances are called phosphors. This type of glowing is sometimes called phosphorescence.
Sometimes glow-in-the-dark objects will only glow very weakly for a short time. Often, you have to place them in a very dark place to see their faint green glow. Newer glow-in-the-dark items may glow more brightly for several hours.
Over the years, chemists have created thousands of chemical compounds that act as phosphors. For glow-in-the-dark toys, manufacturers look for phosphors that can be energized by normal light and that glow as long as possible.