Muons are unstable elementary particles. They are heavier than electrons and neutrinos but lighter than all other particles. They decay via the weak interaction. Because the total of the lepton numbers of the starting and ending particles must be the same, one of the product neutrinos of muon decay must be a muon-type neutrino and the other an electron-type antineutrino. Because charge must be kept the same, one of the products of muon decay is always an electron of the same charge as the muon. So, all muons decay to at least an electron, and two neutrinos. Sometimes, besides these necessary products, additional other particles that have a net charge and spin of zero (e.g., a pair of photons, or an electron-positron pair), are produced.