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Inside a single proton for example, what is the force(s) that keeps the quarks together? Why don't they leave the proton? If they do, how does that even happen?

And maybe an additional sub question: Is it always three quarks inside the protons/neutrons? Why?

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    $\begingroup$ The two word answer is: Quantum Chromodynamics (QCD). en.wikipedia.org/wiki/Quantum_chromodynamics Also, try: frankwilczek.com/Wilczek_Easy.../298_QCD_Made_Simple.pdf‎ $\endgroup$ – Alfred Centauri Oct 11 '13 at 21:14
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    $\begingroup$ I suspect that the drive by down-voter may have done so because (1) it's a very broad question (2) it really doesn't seem that you've done any "homework". There's lot's of introductory information on quarks, gluons, and the force they mediate available online. I recommend that you spend some time digesting some of this and then come back with specific questions about concepts that aren't clear. $\endgroup$ – Alfred Centauri Oct 11 '13 at 22:12
  • $\begingroup$ @AlfredCentauri, thanks for the clarification, I actually didn't know where to look for an answer to the question that crossed my mind while reading something else, and it looks like (QCD) isn't something that can just popup in your mind if you don't ask for it. $\endgroup$ – Tech Support Oct 11 '13 at 22:20
  • $\begingroup$ @TechSupport, please try to read extensively, not just an article or two. Try to find some books by googling: quarks/particle physics/nuclear physics etc. There is a whole lot on the web, as you yourself said. These questions of yours are extremely trivial and it appears as though you are at the very beginning of these things. Therefore, stackexchange is not a good place for you to start. $\endgroup$ – Hasan Oct 11 '13 at 22:56
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Why do proton's quarks stay together? because they have a force between them called the "Strong Force" studied under what's called Quantum Chromodynamics, about which you can read in Wikipedia. The force is a field quantized in the form of bosons called "Gluons", which work like a glue that works the interaction between individuals quarks.

And about your second question: is it always protons/neutrons? Actually not necessarily. The whole story depends on the life-time of the compounds formed by quarks. A simple example can be given from atoms. So the combinations of protons, neutrons and electrons makes atoms. But does that mean that any combination is possible? Of course not. Some combinations are stable, and some are not. Uranium, for example, exists, but isn't stable, and decays to Thorium. Iron exists, but takes a very very long time to decay.

In other words: You may make many many many different combinations of quarks to form either mesons (particles with two quarks) or baryons (particles with three quarks). Some of them are stable that they have a long life-time (like protons), and some others just decay faster (like neutrons, which have around 880 seconds half-life). And some compounds from Quarks decay instantaneously, like Kaons, that live for around $10^{-8}$ seconds.

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