 Consider for example an explanation of the heat capacity of metals and, in particular, of the fact that the heat capacity is much lower than predicted classically (Kittel 2005: 141ff). The explanation appeals to the Pauli exclusion principle and shows how the heat capacity depends on particle statistics. In order to get the correct result for the heat capacity, we need to model free electrons in the metal as satisfying the quantum-mechanical Fermi-Dirac statistics and the exclusion principle. The explanation appeals to the structure of the phase-space available to the electrons.