Entropy

Entropy is a measure of disorder or randomness in a system. The second law of thermodynamics states that in any closed system, entropy always increases over time — order inevitably gives way to disorder. This simple law explains why ice melts, why buildings crumble, why engines are inefficient, and why time flows forward rather than backward. Entropy is one of the deepest laws of physics — it underlies the heat death of the universe (the eventual state of maximum entropy where no useful energy exists), the impossibility of perpetual motion machines, and even the arrow of time itself. Life can temporarily decrease local entropy by importing energy from the Sun.