Water flows down a hill; heat flows from a hot body to a cold one; rubber bands unwind; fluid flows from a high-pressure region to a low-pressure region, and we all get old! Our experiences in life suggest that processes have a definite direction. The first law of thermodynamics relates the several variables involved in a physical process but does not give any information as to the direction of the process. It is the second law of thermodynamics that helps us establish that direction.