Since energy is conserved, why does energy have to be fed to a car to keep it from stopping? Since energy is conserved, why does hot soup cool and ice-cream melt? Why does smoke fill up a room rather than crowding into one corner? Why do re-chargeable batteries eventually die? Why do machines eventually fail?
There are many equivalent statements of the second law of thermodynamics: Isolated systems inevitably become less organised; the usable energy in an isolated system is constantly decreasing; a system naturally attempts to distribute its energy equally among all of its parts; mechanical energy, on average, degrades into heat; heat naturally flows from hot places to cold, equalizing temperatures; isolated machines cannot remain in perpetual motion; entropy (disorder), on average, increases. These statements all express the second law of thermodynamics. Physicists believe that it is the second law of thermodynamics that defines the direction of time, that distinguishes the past from the future.
Great Ideas in Physics, Alan Lightman