 The cornerstone of thermodynamics is the so-called Second Law of thermodynamics. This law describes one of the salient features of the above process: its unidirectionality. We see gases spread—i.e., we see them evolving towards equilibrium—but we never observe gases spontaneously reverting to the left half of a box—i.e., we never see them move away from equilibrium when left alone. And this is not a specific feature of gases. In fact, not only gases but also all other macroscopic systems behave in this way, irrespective of their specific makeup. This fact is enshrined in the Second Law of thermodynamics, which, roughly, states that transitions from equilibrium to non-equilibrium states cannot occur in isolated systems, which is the same as saying that entropy cannot decrease in isolated systems (where a system is isolated if it has no interaction with its environment: there is no heat exchange, no one is compressing the gas, etc.).