Glow-in-the-dark stickers, stars, toys, and clothes, all work by absorbing light and emitting it later. These items contain phosphors, substances such as zinc sulfide that radiate visible light after being energized by natural light. Phosphorescent materials continue to glow after the energizing light is removed. They have electrons that are easily excited to higher energy levels when they absorb light energy.
In phosphorescent materials—such as glow-in-the-dark objects—the excited electrons drop to a lower, but still excited intermediate level and stay there for a period of time before returning to their ground state (original energy level) and emitting the excess energy as visible light.