The Strong Interaction
The strong interaction is responsible for the binding of protons and neutrons in nuclei, and for the binding of quarks inside protons and neutrons. It is generated by the exchange of gluons between quarks. Quarks carry a new quantum number known as color. The strong force acting on quarks inside protons and neutrons prohibits their separation beyond distances larger than the diameters of protons and neutrons. This phenomenon explains why free quarks are not observed, but that quarks are confined in protons, neutrons, and various additional bound states denoted as baryons or mesons, which together are known as hadrons. Gluons carry color as well, and are likewise confined inside hadrons.