Isotopes are atoms of an element with different masses. Most elements exist as mixtures of two or more isotopes. Some isotopes are radioactive (for example, carbon-14), in which case they steadily decay, transmuting into other elements. Others are stable – that is, they are not radioactive and do not change in abundance over time. It has become increasingly clear over the past few decades that analysis of stable isotopes from skeletal remains is a powerful tool for the study of ancient diets, and for studying movements of groups or individuals in palaeopopulations.