Selenium is a chemical element that exists in small amounts in soil, water, and certain foods, and which living things need in tiny quantities to stay healthy. Your body uses selenium to protect cells from damage and keep your immune system working properly. You get the selenium you need from foods like Brazil nuts, fish, and whole grains.

Selenium behaves like a metal in some ways and like a nonmetal in others, which makes it useful in technology. Its special property of conducting electricity better when light shines on it led to important inventions: early television cameras used selenium to convert light into electrical signals, and some modern solar panels use selenium compounds to generate electricity from sunlight.

Scientists initially thought selenium was poisonous, and in large amounts it is. But in the 1950s, researchers discovered that animals and humans actually need trace amounts to survive. Too little selenium can make you sick, but so can too much: it's one of those substances where the dose makes all the difference. A single Brazil nut contains about all the selenium you need for an entire day.