Isotopes are forms of an atom with varying numbers of neutrons that can be identified by their weight. Some isotopes are radioactive and decay over time, while others are stable. Researchers can analyze the isotopic composition of monarch butterfly wing membranes, which does not change after formation, to trace the butterflies' migration patterns and determine where they breed. Isotope analysis is also used in medical diagnosis, pollution tracking, dating fossils, and determining past climates by analyzing oxygen isotope ratios trapped in ice and ocean organisms.