Both natural and artificial mesons are unstable. The smallest of these are the pions. They come is 3 charged states: +1, -1, and 0. Positive ones are made of 1 up quark and 1 anti-down quark. Negative ones are made of 1 anti-up and 1 down quark. On the other hand, the neutral pions are made either of up and anti-up or down and anti-down with equal probabilities as coherent superposition of the two states. The charged ones have a mass of 139.57 MeV/c while the neutral mass is 134.98 MeV/c, a difference of 4.59 MeV/c. This meson mass disparity is opposite that of nucleons. For nucleons such as protons and neutrons, it is the zero charged neutrons that are heavier than the charged protons.