 Back in 1964, a quark model was proposed by Murray Gilman and George Zweig to help explain protons, neutrons, and the wide variety of newly discovered heavy particles like pions and caons and others. The discovery at Slack in 1969 that the proton has three parts constituted evidence that quarks were real. One of the key things to remember about the theory is that quarks are so strongly bound together that it is impossible to study one on its own in order to determine its properties. This means that all we know about quarks is derived from the properties of the particles that bound quarks create. We call particles made of quarks hadrons, meaning heavy. In studying hadrons we find two kinds. Those with two quarks are called mesons, and those with three quarks are called baryons. All quarks have a spin of one-half, and the sum of quark charges needs to give us the charge of the hadron. The baryons we've seen so far are the proton and the neutron. Quark theory has it that there are two flavors of quarks that make up these baryons called the up quark and down quark. Protons have two up quarks with one down quark, and the neutron has one up quark and two down quarks. In order to get the correct charge of the proton and the neutron, the up quark must have a positive charge that is two-thirds the charge of an electron. The down quark must have a negative charge that is one-third the charge of an electron. The down quark must also have a little more mass than the up quark for the neutron to have a little more mass than the proton. Note that the sum of the quark masses falls far short of the mass of the baryon. This indicates that there is a lot more going on inside the proton than we've seen so far. The up and down quarks are the lightest and most stable quarks, and all other quarks will decay into these two over time. The two masons we've seen are the pion and the kaon. The positively charged pion has an up quark and an anti-down quark. The negatively charged kaon contains an anti-up quark and a third kind of quark called the strange quark. It was needed to explain the length of time it took the kaon to decay. In addition to the up, down, and strange quarks, we have discovered the charm, top, and bottom quarks for a total of six. One of the key rules seems to be that they can only combine in combinations of two or three as long as the sum total of charge always equals the charge of an electron or a proton or zero. So armed with quarks, physicists intensified their search for some of the three quark particles predicted by Gilman and Zweig's theory, the lambda, psi, and omega baryons.