Radioactive!

One of my newest toys is a Glowring. Within the protective plastic housing, the radioactive tritium gas reacts with the phosphor coated glass vial. This creates cold energy which produces a low level light source which will glow, maintenance-free, for 10 - 20+ years. Not legal for sale in the US, but you can buy a "novelty key chain" on eBay. They come in pink, green, and blue, and are completely safe.