Carbon-14 forms in the atmosphere through cosmic rays interacting with nitrogen-14, creating a small fraction of carbon-14 in living organisms. When an organism dies, its carbon-14 decays back into nitrogen-14 at a known rate, called a half-life (5,730 years). By measuring the remaining carbon-14 in fossils, scientists can estimate their age, helping us understand the history of life on Earth. Created by Sal Khan.