When any matter is divided into smaller and smaller pieces, eventually all you are left with—when it cannot be divided any further—is a particle. Currently, there are 12 different known elementary particles, which in turn are made up of quarks and leptons, each of which come in six different flavors. These flavors are grouped into three generations—each with one charged and one neutral lepton—to form different particles, including the electron, muon, and tau neutrinos. In the Standard Model, the masses of the three generations of neutrinos are represented by a three-by-three matrix.