A quark is a subatomic particle that is one of the fundamental constituents of matter. Quarks are the building blocks of hadrons, particles that contain an even number of valence quarks. There are six types, or “flavors,” of quarks: up, down, strange, charm, top, and bottom. In addition, quarks come in three “colors”: red, green, and blue.

Quarks interact with each other via the strong force—the same force that holds protons and neutrons together in an atom’s nucleus—and they are held together by gluons. The strong force is mediated by bosons called gluons. Gluons bind quarks together to form composite particles called hadrons—the most familiar being protons and neutrons (which consist of two up-type or down-type quarks, respectively).

The Standard Model predicts that all elementary particles are either bosons or fermions; however, there is currently no known way to explain why this should be so from the first principles (although several theories have been proposed). In addition to the six flavors mentioned above, there could theoretically be a seventh flavor: the top squark (a supersymmetric partner particle for the top quark). However, this has not yet been observed experimentally.