Neutrinos are fundamental particles, meaning one of the types of tiny particles that make up everything. In fact, neutrinos are the most abundant particles with mass in the universe. If you hold out your thumb and blink, almost 100 billion neutrinos passed through your thumbnail in that time. However, another important property about neutrinos is that they don’t like to interact much. Even though trillions of neutrinos pass through your body every second, in your entire lifetime only one or two will stop and interact.
Neutrinos are incredibly light and come in three kinds (called “flavors”), named after the particles they produce when they do interact. There are electron, muon, and tau neutrinos. Neutrinos are unique among fundamental particles because they change flavors as they travel—this is one of the reasons they are so exciting to scientists.