This article is about energy in terms of natural sciences. For other uses, see Energy (disambiguation). Lightning is the electric breakdown of air by strong electric fields (from electric potential energy to mechanical energy of the random motion of air molecules (heat), and to light). Lightning is the electric breakdown of air by strong electric fields (from electric potential energy to mechanical energy of the random motion of air molecules (heat), and to light). In physics and other sciences, energy (from the Greek ενεργός, energos, "at work") is a concept, a quantifiable attribute of physical systems. It is not an object or a substance, but it is quantifiable in ways which show that it cannot be created or destroyed. The units used to quantitate energy are the same as those used to define work in physics. Many forms of energy are defined in the natural sciences, e.g. kinetic energy, potential energy, electrical energy; chemical energy etc.; any one of them can be transformed into another form, often through a process that involves mechanical work. However, the transformation from one form to another is not always total, it is limited by the second law of thermodynamics. A fraction of the energy remains in the form of thermal energy in many such transformations.