Quarks are hard, point-like objects found within the nucleus of an atom. When Quarks combine in threes, they form compound particles known as Baryons. Protons are probably the best-known of the Baryons. Sometimes, however, Quarks interact with corresponding anti-Quarks, which have the same mass but opposite charges. When this happens, they form Mesons. Mesons often turn up in cosmic rays. Mesons, Baryons, and other kinds of particles that take part in strong interactions are called Hadrons. This classification of particle physics has remained virtually unchallenged... Until now.