 Next, we'll talk about the strong force. The strong force acts between quarks, and as you might recall, only acts over very small distances, but 10 to the minus 15 meters, on the order of the size of the atomic nucleus. The exchange bosons for the strong force are called gluons. Like photons, they're massless, and they're only exchanged between quarks and anti-quarks. This time, though, the exchange process is a bit more complicated, because there's the third property, known as color, that determines whether certain quarks can be drawn together via the strong force. There are three possible colors that quarks can have, red, blue, or green. The baryon that each set of quarks builds must be colorless, which means you can have one of each type of quark, a red, a blue, and a green. This is an analogy to light. If you combine red, blue, and green light, you get white. This constrains how you can make baryons. Or you can have a quark-anti-quark pair of the same color. This constrains how you can make mesons. Now, quarks don't really have color in the visual sense. The color here is a visual tool we can use to think about which quarks can bind together. The theory that underlies this explanation is known as quantum chromodynamics, or QCD. And the color is basically another quantum number. It's a handy one, too. It allows us to put quarks, which are fermions, and therefore can't live in the same quantum state, in configurations that might be impossible otherwise. These impossible configurations were, in fact, how color was discovered in the first place. But that's the story for another time, because we have one more force to go.