What are glow-in-the-dark crayons?

Glow-in-the-dark crayons glow because a substance known as phosphor is added to them when they are manufactured. Phosphor possesses photoluminesce, which means that when energized, it can radiate visible light.

The two most common phosphors are zinc sulphide and strontium aluminate; of the two, strontium aluminate is the stronger phosphor. To be able to glow, the crayons must first be energized or charged. As with most glow-in-the-dark toys, charging can be done by shining a light on the crayons for a few moments. Stored within the phosphor molecules, the photons of light release at a slower rate than when originally charged.