You probably have a few smoke detectors throughout your house, but did you know that many designs of smoke detectors are radioactive? Well in this video we explore how smoke detectors work, and how they use the radioactive isotope Americium-241 to detect smoke. We'll also explore how Geiger counters work, and then how atoms of high atomic numbers are counterintuitively small. Smoke detectors are perfectly safe, and save lives. So make sure that they work on a regular basis.