To show how isotopes can be used to locate elements in the body, Dr. Robert Ballentine drinks radioactive iodine 131 at the start of the program. Later he uses a Geiger counter to show the level of radioactivity in his stomach and thyroid, where it accumulated. Such a tracer can be used in biological research and as a medical diagnostic tool. Dr. Ballentine further describes the characteristics of isotopes, the Brookhaven Pile at Brookhaven National Laboratories, and how isotopes are obtained.