These are UV sensitive beads that change color when exposed to light in the wavelength range 300 to 360 nm. The sun or flourescent black lights work well. The dye molecules consist of two large, planar, conjugated systems that are orthogonal to one another. No color is produced in this state. However, UV light supplies the energy to make the systems planar and able to absorb visible light, producing color. Heat from the surroundings is sufficient to switch the molecule back to the orthogonal structure.