 On the face of it, the Zeroth, First and Third Laws are straightforward to understand. The Second Law, however, is not so “obvious” as it may at first glance appear. It may seem that, because nothing can escape from a black hole once it has entered, black holes can only grow larger or, at least, stay the same size if nothing further falls in. This assumes, however, that increased mass always yields increased surface area as opposed to some other measure of spatial extent. Surprising as it may sound, it is indeed the case that, although nothing that enters a black hole can escape, it is still possible to extract energy (i.e., mass) from a spinning black hole, by means of what is known as the Penrose process (Penrose and Floyd 1971). It is therefore not obvious that one could not shrink a black hole by extracting enough mass-energy or angular momentum from it. It also seems to be at least possible that a black hole could shrink by radiating mass-energy away as gravitational radiation, or that the remnant of two colliding black holes could have a smaller surface area than the sum of the original two. 