Although defining energy is one of the most difficult things in physical science, in its simplest definition, energy is what makes it possible for everything that happens around us to occur. If cars are moving today, if your house is getting warmer, if factories can produce, flowers are blooming and the sun can warm you, the cause is energy. In more technical terms, energy is the name given to the ability of materials to do work. Substances can change their current physical state only by using energy. Observable motion energy is called kinetic energy. There is also energy that is stored and ready to be used in the system even though it is stationary. This is called potential energy. For example, a ball standing still on a ladder has potential energy because when the obstacle is removed it can move down by gravity without doing anything else; When it starts down the ladder and gains speed, its potential energy turns into kinetic energy.