Phosphorescence is always an engaging demonstration. A high-energy photon source, such as a violet laser or UV black light, shines on a phosphor (screen, mineral, toy, etc.) and then the light is reemitted slowly at a color of lower energy. We notice that the intensity of the emitted light dims with time. Since the intensity of the light emitted depends directly on the number of atoms or molecules that have not yet emitted, this dimming can be modeled as an exponential decay very similar to those students encounter while studying radioactivity, but with the advantage of being perfectly safe. In this article, I present a quantitative method for measuring the half-life of this decay and go further to attempt to explain why temperature can have such dramatic effects.