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The trefoil symbol is used to indicate radioactive material.

Radioactive decay happens to some chemical elements. Most chemical elements are stable. Stable elements are made up of atoms that stay the same. Even in a chemical reaction, the atoms themselves do not ever change.

In the 19th century, Henri Becquerel discovered that some chemical elements have atoms that change over time. In 1898, Marie and Pierre Curie called this phenomenon radioactive decay. Becquerel and the Curies were awarded the Nobel Prize in Physics for this discovery, in 1903.