No isolated quark has ever been observed: quarks, as well as gluons, seem to be bound permanently together and confined inside hadrons. This phenomenon is known as confinement. It relates to the fact that, in addition to an electric charge (which is a fraction of the electron's one), each quark has a colour charge, which can only take 3 values (red, blue or green). For normal conditions of temperature and density, quarks only exist within hadrons - aggregates of two or three quarks that are colour neutral (i.e. white). Confinement is also the fundamental property that gives each hadron a mass much larger than the sum of the masses of its constituents: in fact, confinement generates about 99% of the nuclear mass.