This means that gluons react with quarks but not with leptons. Leptons consist of three types of charged particles and a neutral "neutrino" for each type of charged particle. Of these 6 lepton particles, only 4 are stable in the universe: electrons, electron neutrinos, muon neutrinos and tau neutrinos. The other leptons, muons and taus, are more massive than their first-generation cousins, electrons, and will therefore decay into a more stable state through the weak force. Quarks have two different types and also three flavors of each type. There are quarks with charge -1/3 and quarks with charge 2/3. Of these, the only quarks found in fairly stable particles are the up quark, the down quark and the strange quark. The other quarks (charm, bottom, top) are very unstable and decay very quickly after being created. Quarks are always accompanied by gluons and are always found in sets where their total color charge is zero. Quarks are what make up hadrons and mesons. Hadrons are particles composed of quarks and gluons. Baryons and mesons are different types of