Conversely, there are situations in which the enthalpy change is zero and the entropy, expressed -TD S, is all-important. This is true for the universe as a whole, or for any part of it that constitutes an isolated system, which interchanges neither mass nor energy with its surroundings. One meaningful example in recent years is a sealed space capsule.

Every operation that the astronauts perform increases the disorder in the capsule, thereby decreasing the overall free energy, or capacity to do still other things: AG =-TDS. The space capsule and everything in it literally run down, like the universe, but on a far shorter time scale. If the capsule is not opened and provided with new supplies of free energy, eventually it and all of its contents will degrade to the state of high entropy that conventionally is known as death.