Radioactivity is when certain atoms naturally break apart and release invisible energy and particles. This process happens on its own, without anything triggering it, like a clock that winds down by itself.

Most atoms are stable and can stay the same for a very long time, but radioactive atoms are unstable. They throw off tiny pieces of themselves until they transform into different, more stable atoms. When this happens, they release radiation: invisible rays of energy that can travel through air, water, and even solid materials.

Scientists discovered radioactivity in the 1890s while studying uranium, a heavy metal. Marie Curie and her husband Pierre won the Nobel Prize for their groundbreaking research on radioactivity.

Radioactivity can be dangerous because the radiation it produces can damage living cells, but it's also incredibly useful. Doctors use small amounts of radioactive materials to take pictures of the inside of your body or to treat cancer. Nuclear power plants use radioactivity to generate electricity for millions of homes. Scientists use it to figure out how old fossils and ancient artifacts are.

Some materials stay radioactive for just seconds, while others remain radioactive for thousands of years. The word radioactive describes something that gives off this kind of radiation.