A neutrino is an almost massless particle created in abundance in the nuclear fusion reactions in the hearts of stars. They usually zip through all forms of matter without interacting, making them difficult to detect. (In fact, trillions of them pass through your body every second.) Physicists have built special detectors deep below ground. Neutrinos occasionally zap atoms and molecules in the detector chambers, producing brief flashes of light. From these reactions, physicists can determine the origin of the neutrinos. Astronomers have detected neutrinos produced in supernovas and other powerful events, enhancing our understanding of these objects.