In 1991 in the Alps, near the border between Italy and Austria, two hikers found a preserved body in a melting glacier. The oldest known natural mummy in Europe, the corpse sparked curiosity throughout the scientific community. He was nicknamed Ötzi the Iceman, and everyone wanted to know what kind of life he lived 5,000 years ago.

Using isotope science, scientists were able to draw conclusions about where Ötzi might have lived, both as a child, and as an adult. This kind of forensic work is not the only use for isotopes. From the food we eat to catching athletes that cheat or understanding planets in space, isotope science has a vast array of applications. However, before we can understand any of these applications we must first understand some fundamental ideas about isotopes. After exploring what isotopes are, how we know they exist, and how they determine the atomic weights of elements, we will return to the story of Ötzi.