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For this reason, much of what is known about quarks has been drawn from observations of hadrons. The increase in M (p) for pa > 1 is an indication of the difficulty of accurately subtracting off the tree-level mass function for this definition of the improved propagator. Owing to a phenomenon known as color confinement, quarks are never found in isolation they can be found only within hadrons, which include baryons and mesons, or in quark–gluon plasmas. Quark is a type of fresh cheese originally from Central Europe. Some versions have skimmed milk powder added and can be rather gritty. Soft and moist, like a cross between yogurt and fromage frais, it should taste lemon-fresh. All commonly observable matter is composed of up quarks, down quarks and electrons. Quark can be made from whole, skimmed or semi-skimmed milk or even buttermilk. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. A quark is a type of elementary particle and a fundamental constituent of matter.