An unstable nucleus can decay spontaneously, emitting radiation and sometimes changing element. Decay is random: the time of an individual nucleus’s decay cannot be predicted. Activity is the number of decays per second, measured in becquerels; large samples show a predictable average decay pattern. Example: Explain why warming a source does not provide a reliable way to control its decay. Ordinary temperature changes do not determine when individual nuclei decay. Use clear labels. Use evidence.