This law was put forward by Sadi Carnot, who was trying to establish the most efficient engine possible. He observed that although energy may be conserved during a physical change, some of the energy will be rendered unavailable for work. This became the basis for the second law of thermodynamics. The variable for the second law of thermodynamics is called entropy. Entropy in a mathematical sense can be defined as being equal to the energy change of a system divided by the temperature of the system.
All systems attempt to maximize their entropy.
This means that they inevitably tend towards a simple final state or equilibrium. Entropy always increases with time, but the reverse never occurs. This rule implies that decay is a universal trend in nature. In attempting to reach their entropy, mountains and stars wear away and living organisms return to inorganic matter in death. The essential characteristic of the universe is the one-directional passing of time.