Questions II

I decided to break up the questions, to try to publish a little more often. I might also save some for later. Questions and answers after the jump.

How did quarks come to be discovered? 
Back in the 1960's, physcists were running into the same problem that used to plague chemists: there were a whole lot of fundamental particles, and more kept being discovered. This was still in the era when we thought protons and neutrons were fundamental particles, as were pions and kaons and all the mesons and baryons. Some very smart physicists started arranging these particles by various fundamental properties, mainly charge and strangeness, a property we can discuss later. To explain the relationship between these many particles, Murray Gell-Mann and George Zweig introduced the idea that the particles we were observing were in fact made up of smaller constituents. The net properties of these constituents would then be the properties of the larger particle. At first, most physicists just assumed that these constituent particles weren't real, and were instead just a mathematical device to understand particles.
However, in the late 60's, the Stanford Linear Accelerator started doing studies of what happens when one bombards a nucleon with a high-energy beam of electrons. The result was that you got to see what makes up the nucleon. Rather than just being a point particle or having no internal structure, it was demonstrated that the nucleons were themselves composed of smaller particles that seemed to be point-particles. While physicists hesitated to identify the innards of the nucleons as quarks, follow-up experiments would show that the constituent particles behaved just like you would expect certain quarks to behave. So, the physics community finally accepted quarks as real, and as the constituents of the nucleons.
 
Why are neutrons neutral? 
Neutrons are neutral because they contain 2 down quarks and an up quark: the sum of the charges is 0. Probably the better question to ask is: if we see the proton (uud=+1) and the neutron (udd=0), why don't we see some sort of baryon that satisfies (uuu=+2) and (ddd=-1)? The answer to that question seems to lie deeper in the math behind the theory. I'll try to wrap my head around it before making any firm claims of comprehension. Let's just say that there's an additional property to each quark that has to satisfy specific requirements, and while 2 ups and a down and 2 downs and an up satisfy those conditions, 3 of either type of quark doesn't.
Such particles do exist, they are just at much higher masses, and therefore less stable. I don't quite understand them yet (the so-called Delta baryons).