 For those who know classical thermodynamics, the formal analogy between its laws and the laws of black hole as stated should be obvious. (For exposition and discussion of the laws of classical thermodynamics, see, e.g.: Fermi 1937 for a less technical, more physically intuitive approach; Fowler and Guggenheim 1939 for a more technical and rigorous one; and Uffink 2007 for a more historically and philosophically oriented one.) One formulation of the Zeroth Law of thermodynamics states that a body in thermal equilibrium has constant temperature throughout. The First Law is a statement of the conservation of energy. It has as a consequence that any change in the total energy of a body is compensated for and measured by changes in its associated physical quantities, such as entropy, temperature, angular momentum and electric charge. The Second Law states that entropy never decreases. The Third Law, on one formulation, states that it is impossible to reduce the temperature of a system to zero by any physical process. Accordingly, if in the laws for black holes one takes ‘stationary’ to stand for ‘thermal equilibrium’, ‘surface gravity’ to stand for ‘temperature’, ‘mass’ to stand for ‘energy’, and ‘area’ to stand for ‘entropy’, then the formal analogy is perfect. 