The historical development of the classical postulates of the second law of Thermodynamics can be traced back to the book by Sadi Carnot, "Reflections on the motive power of fire." While unique in its own right and in some sense revolutionary, the book starts with an analogy between heat engines and waterwheels. Waterwheels were common engines of the time, whose workings had been previously described in a scholarly fashion by Lazare Carnot (Sadi's own father) however, power-producing steam engines were new propositions, being built empirically with no underpinning theory. It is obvious that the father-son relationship must have influenced Sadi's perception and possibly suggested the rendition of the new and emerging steam engines in terms of accepted mechanical descriptions. Sadi Carnot's famous book starts from the premise that one can compare the workings of a steam engine with those of a waterwheel. The analogy, although fundamentally flawed and today largely forgotten, set the ground for the development of modern thermodynamic theory. This paper revisits the hydraulic analogy, not as a theoretical result but as a suggestion of a modern and enlightening way of introducing the basic concepts of irreversibility, efficiency and entropy generation to students embarking into the mysteries of the second law of Thermodynamics.