What is the First Law of Thermodynamics?

The First Law of Thermodynamics is that energy cannot be created or destroyed, it can only change from one form to another. For example, a boulder perched at the top of a hill has gravitational potential energy but then when it is pushed and begins to roll down the hill some of the potential energy is changed to kinetic energy. In this example no energy has been created or destroyed. The kinetic energy was always present just in the form of potential energy. Therefore, the total amount of energy in the world remains constant.