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A pion (π) is a type of hadron, i.e., a type of a composite particle made up of quarks. More specifically, it is a type of meson, a hadron made of exactly two quarks. In contrast, protons and neutrons are hadrons made up of three quarks each. Pions decay extremely rapidly (into multiple leptons or photons), but are involved in the strong force within atomic nuclei, and are relevant in some of the extreme circumstances found in astrophysics, e.g., the innards of neutron stars (pion condensates), and the unfolding of the very early universe.