Quarks are one of the two basic constituents of matter in the Standard Model of particle physics. (The others are leptons.) Antiparticles of quarks are called antiquarks. Quarks and antiquarks are the only fundamental particles that interact through all four of the fundamental forces. An important property of quarks is called confinement, which states that individual quarks are not seen because they are always confined inside subatomic particles called hadrons (e.g., protons and neutrons); an exception is the top quark, which decays so quickly that it does not hadronize, and can therefore be observed more directly via its decay products. Confinement began as an experimental observation, and is expected to follow from the modern theory of strong interactions, called quantum chromodynamics.