Welcome to brain Stuff, the how stuff works podcast that can make you smarter in sixty seconds. Hi Marshall, brain You've probably seen all kinds of glow in the dark toys. You hold them up to a light and then they glow in the dark for half an hour. Occasionally you'll see something glowing, but it doesn't need charging like this. The most commonplace is on the hands of expensive watches. In these products, the phosphor is mixed with a radioactive
element and the radioactive emissions energized the phosphor continuously. Does it last forever? Well, no, but it can last a pretty long time. In the past, the radioactive element was radium, which has a half life of sixteen hundred years. Today, most glowing watches use a radioactive isotope of hydrogen called tritium, which has a half life of about twelve years, or prometheum, a man made radioactive element with a half life of around three years. Do you have any ideas or suggestions
for this podcast? If so, please send me an email at podcast at how stuff works dot com. For more on this and thousands of other topics, go to how stuff works dot com.