The look around shows that plants grow and that animals are moved, that the machines and tools perform more tasks. Energy is a property associated with objects and substances and manifests itself in the transformations that occur in nature. Energy manifests itself in physical changes, for example, to raise an object, transport it, deform or heat it up. Energy is also present in chemical changes, as in burning a rozo of wood in the decomposition of water by electric current. Man, throughout its history has made through its own physical exertion activities that consumed energy, relying on the domestic animals such as horses, oxen, etc. Until the arrival of the industrial revolution, the use of mechanical systems to provide power was limited to the mills of wind or water. Another application of these technologies for the realization of works was little performance.
Sources of energy, the first and most important of those used by the man was firing, due to the abundance of forests which proliferated all over the world. Other point sources were only used there where were accessible, such as petroleum, coal or asphalt surface seepage. In the middle ages he began using firewood to make coal plant with whose ores metals were obtained, and which would subsequently be replaced by coal in the principles of the industrial revolution.