Beginning with a not very rigorous definition of work, Asimov goes on to discuss kinetic and potential energy and is at his best when retracing the reasoning of scientists like Count Rumford, who figured out the relationship between heat and energy while wondering why cannon-boring made the metals so hot; Julius Mayer who posited the conservation of energy; and Joule and Helmholtz who figured out how to demonstrate the theory experimentally. The trail leads up through Clausius' postulation of entropy and the first discoveries in nuclear energy. Such generalized concepts as "energy evening out" may require a leap of faith at this level, but like. . . Electricity (1973), this is an area where a clear, non-mathematical explanation should pay its way.