Neutrinos are elementary, subatomic particles, first postulated by W. Pauli in 1930 and first detected by F. Reines and C. L. Cowan in 1956. They carry no electric charge and interact with other particles only via the weak force. Neutrinos come in three flavors, as distinguished by the weak force. The three flavors are electron neutrino, muon neutrino, and tau neutrino. Other known elementary particles, such as quarks and charged leptons, also come in three flavors that are successively heavier in mass; however, the situation for neutrinos is less definitive. Until recently, all that was known about neutrino mass was that neutrinos must be very light if they had any mass at all. Theoretically, within the standard model of particle physics, neutrinos were even considered to be massless.