‘Identical outsides … different insides’ describes the isotopes of hydrogen (protium, deuterium, tritium) and carbon (carbon-12, carbon-13, carbon-14). The isotopes exist due to an extra particle (neutron) or two in the element’s nucleus, which adds extra mass to the atom. Tritium and carbon-14 are both unstable and undergo radioactive decay. There are four types of radioactive decay: α, β, γ (alpha, beta, gamma), and spontaneous fission. Radioactive decay is the process whereby the nucleus of an unstable atom loses energy by emission of ionizing radiation. The rate of the radioactive decay is measured by the ‘half-life’—the time needed for half the radioactive isotopes of a substance to decay.