Radioactive carbon dating is the way we measure the age of formerly living things. Every living thing takes in carbon either in the food that it eats or absorbing it from the carbon dioxide in the atmosphere. Most of the carbon from the atmosphere is in the stable form carbon-12, but one in a million molecules of carbon in the atmosphere involve carbon-14, the radioactive form. The radioactive form of carbon, carbon-14, decays with a half-life of 5700 years to nitrogen-14. When any living thing dies it stops ingesting carbon, and so the amount of carbon-14 is fixed at a particular value. The rate at which it changes to nitrogen-14 therefore gives the age of the object. Carbon dating has been used to date the age of the oldest trees on Earth and has been used, for example, to show that the Shroud of Turin was a fake. It's routinely used to measure the age of formerly living things.