Theoretical physicists tend to use units of length, energy, and temperature interchangeably. Energy is related to temperature through the expression *E* = *kT*, where *k* is Boltzmann's constant, so high energies correspond to high temperatures. Length and energy have a very direct relationship in quantum mechanics. The energy of a particle is related to its wavelength when it is described as a wave. The proton is a good example—its mass gives an energy (when multiplied by *c ^{2}*, the speed of light squared), which is roughly the binding energy of the quarks inside it, and its mass also gives a size (when Planck's constant,