discussion of thermodynamic energy and its conservation in Lectures in Physics begins with the premise that it is a numerical quantity that does not change when one or many alterations in the system occurs. His heurism for energy and its conservation involves the premise that Dennis the Menace has 28 indivisible blocks, a number which his parents find constant at the end of every day of play. If one day a count yielded 27, an investigation would reveal that a block could be found elsewhere, say under the rug. If at the end of the day, the count was 29, the extra one had to come from somewhere else, perhaps Dennis’s playmate Bruce. If Dennis locked some of his blocks in the toy box and threw some into a bathtub of dirty water and (1) A block weighed three ounces; (2) The box alone weighed 16 ounces; and (3) Each block raised the water level of 6 inches by one fourth of an inch, then this metaphoric energy relation can be expressed: (Sisake weanj+ (weight of box)-16 ounces + (height of water)-six inches = constant (28) 3 ounces 1/4 inch Feynman notes that this representation of an energy relation, computed as a number of blocks, will always remain the same. If there were no blocks in sight, and one used this energy conservation relation with blocks as units of energy, we find no blocks as such in the expression at all. The abstract and formal idea of energy in physics first arose in mechanics and was generalized to electrostatics and electrodynamics. If one idealizes these systems, eliminating real world factors such as friction, temperature gradients, temperature dependence of the properties of materials, viscosity, hysteresis and other nonlinear behavior, then the energy conservation law says that in an isolated and interacting set of systems, the sum of the energies of the several systems remains constant. If, on the other hand, a system interacts with its surroundings, not isolated and interacting, then the increase in the energy of the index system is equal to