Many semiconductors exhibit the property of phosphorescence, the phenomenon whereby a material excited by visible or ultraviolet radiation glows (i.e., emits visible light) for a substantial time after the excitation source has been removed. One phosphorescent semiconductor commonly used in glow-in-the dark toys and emergency signs is copper-doped zinc sulfide (ZnS:Cu), a material that produces a yellow-green glow. The study of this light emission is a suitable introduction to kinetics and to semiconductors. Intensity data were recorded for a Glow-Yo yo-yo, an Amazing Glow Ball, Hanovia P-1000 pigment, and Glo-Vinyl Tape by spectrofluorophotometer and also by using a CdS photocell and ohmmeter. The emission of the ZnS semiconductor is not across the band gap, but involves interband traps associated with the Cu doping material. The phosphorescence decay of ZnS:Cu fits a second order equation, consistent with recombination of relatively equal populations of holes and electrons (n ~ p). Arrhenius data show the delocalized electrons in the conduction band must overcome a small but measurable activation energy of about 15 kJ/mol to fall back to the localized copper-site holes for the recombination that produces phosphorescence.