 When confronted with a vast number of observations, a first step to understanding is categorization. We try to find the similarities and differences between things. When it comes to elementary particles, we have already made some distinctions. We have the small light particles, like the electron in the neutrino, called leptons. And we have the quarks that make up the heavy particles, like protons and neutrons, called hadrons. Everything we see around us is made of these three stable elementary particles, the electron, the up quark, and the down quark. A particle's spin is another key distinction that helps us categorize all these particles. You'll recall that electrons and quarks have spin one-half and therefore follow Pauli's exclusion principle. Photons have spin one and do not follow the exclusion principle. The statistics that describe spin one-half particle behavior in large groups was developed by Enrico Fermi and Paul Dirac. They are called fermions after Mr. Fermi. The statistics that describe spin one particle behavior in large groups was developed by Satyandronath Bose and Albert Einstein. They are called bosons after Mr. Bose. You can imagine that large groups of particles that can't fit into the same quantum state will behave differently than particles that can. In an energy well, the bosons all sit in a condensate at the bottom. The fermions arrange themselves in a hierarchy, like electrons and an atom. For example, a beam of photons can be made to have the same quantum state. This is how a laser works. On the other hand, the inability of electrons to fit into the same quantum state creates an outward pressure that halts a star's collapse and creates white dwarfs. In this segment, we also covered the muon, a higher energy version of the electron. At even higher energies, another electron-like particle called the tau was discovered in 1975. When these leptons decayed, their neutrinos were slightly different. So we have two additional neutrinos, the muon neutrino and the tau neutrino, to go along with the ubiquitous electron neutrino. Experimental evidence indicates that decay rates for these particles are different. We have the Gen 1 particles that are stable. They do not decay. In addition, we find that Gen 2 particles decay slower than Gen 3 particles. This gives us one more organizational category to go along with the heavy versus light and the integer versus non-integer spin. So here we see the beginnings of the standard model of particle physics. We'll finish developing this model in our final segment on the Higgs boson.