The smallest subatomic particles are quarks and leptons. There exist six known types of quarks and six known types of leptons. These quarks and leptons make up fermions (which can be visualized as the particles of matter). Examples of quarks include the up quark, the down quark, the charm quark, the strange quark, the top quark, and the bottom quark. A proton is made up of one down and two up quarks whereas a neutron is made up of one up and two down quarks. Examples of leptons include electrons, muons, tau, electron neutrinos, muon neutrinos, and tau neutrinos. As per the string theory, quarks and leptons are made up of energy that vibrates in the shape of a string. The specific vibrations of these strings afford different subatomic particles.