Some physicists have pointed out that we do not know what energy is. If there is no clear idea of energy, teaching the concept must be a problem. There has been much criticism concerning explanations of energy in textbooks and research literature about studentś misconceptions is ample. In the History of Science, one learns that Mayer and Joule discovered energy. A study of their experiments, calculations and interpretations of phenomena shows that they did not find anything which is indestructible and transformable but rather a methodology of dealing with phenomena. How to understand energy thanks to their works is the central subject of the present paper.