When a thing is radioactive, it is sending out radiation: energy you cannot see, coming from inside atoms (the tiny pieces everything is made of). That happens when the centre of the atom is not steady and throws off energy or tiny bits. Strong radiation can harm living things — it can make a person sick, hurt the body slowly over years, or, in a very large amount, kill. A weak amount is around us anyway, from the sun and from some rocks, and that is usually nothing to fear. Scientists and hospital staff use the word carefully. In ordinary talk people sometimes stretch it, as if it only meant “poison” or “keep away,” and they even use it as a joke about a subject nobody wants to touch. That joke is not the science meaning. And though the letters look like radio, the music machine, this word is about rays from atoms, not about songs.