Sometimes isotopes are unstable and the nucleus of the atom breaks down. This break down gives off bits of the nucleus or energetic rays.
This causes radioactivity.
Carbon-14 is a well-known radioactive isotope. It is used to date organic matter.
Always a tiny fixed proportion of carbon-14 in living matter.
When the living material dies the carbon breaks down and the reactivity levels fall slowly.
As the half-life of carbon-14 is known, scientists can work backwards and work out how long it took for the level of radioactivity to fall from the level it was when the organism was living to the level it is now.
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