Why do some elements glow in the dark?

The answer

Some radioactive elements like radium and tritium emit radiation that excites nearby phosphorescent molecules, causing them to glow. Radium's eerie green glow was first observed by Marie Curie, who kept glowing radium salts by her bedside. However, most everyday 'glow-in-the-dark' products use phosphorescent materials like strontium aluminate, which absorb photons and slowly re-emit them over hours. These materials aren't radioactive; they simply store light energy and release it gradually through quantum mechanical transitions in their crystal structure.