  In this paper the analogy between a thermal engine and a waterwheel is
developed in details, showing that the analogous of the flow of water in an
hydraulic engine is the flow of entropy in a thermal one. This analogy mat
serve to analyse in details the aspects of a thermal engine that are quite
distant from pupils' intuition. The discussion is complemented by an
illustration of the working of thermal and hydraulic engines at the microscopic
level, thus furnishing a view of what heat and work are at this level, and
introducing the statistical and information aspects of entropy. Both hydraulic
and thermal non-ideal engines are also discussed.
