Quantum chromodynamics (QCD) is the theory of quarks and gluons. Quarks are the fundamental constituents of nuclear particles such as the proton and the neutron. Isolated quarks have never been observed, and it is thought that they never will be. Quarks interact via forces mediated by particles called gluons. These forces have the unusual property that, although they appear feeble in high-energy collisions, they become very strong at the energies typical in nuclear particles. This property, called asymptotic freedom, explains why the constituents of protons appeared almost free in early high-energy collision experiments and yet have never been observed. They are permanently confined inside protons, neutrons, and the other hadrons (particles that contain quarks and gluons).