More years ago than I would care to admit, a class in thermodynamics was required as part of my undergraduate course of study in electrical engineering. All of us who were both engineering and premedicine students that year chose a particular class given by the chemistry department, since we could satisfy both an engineering requirement and a prerequisite for medical school at the same time, a form of “double-dipping” that was hard to resist. In short order, we realized that learning chemical thermodynamics was not a trivial undertaking, and many of us were fortunate to survive that class with our grade point averages more or less intact. Fast-forward 30-something years, and chaos theory has become the darling of applied mathematics, and a characteristic of chaos called entropy, which is dangerously close to a property taught in that dreaded thermodynamics class, is being used to describe all sorts of physical systems including biological systems. A case in point is the article appearing in this issue of CHEST (see page 80) by Burioka and colleagues reporting on measurements of the approximate entropy (ApEn) of respiration during wakefulness and sleep.