Enlarge / Particle tracks from the LHCb detector. (credit: Brookhaven National Lab) The quark model was an intellectual revolution for physics. Physicists were faced with an ever-growing zoo of unstable particles that didn't seem to have a role in the Universe around us. Quarks explained all that through an (at least superficially) simple set of rules that built all of these particles through combinations of two or three quarks. While that general outline seems simple, the rules by which particles called "gluons" hold the quarks together in particles are fiendishly complex, and we don't always know their limits.