So when my dad first explained the fundamental idea of thermodynamics to me, that entropy never decreases, he pointed out the odd fact that according to basically all other laws of physics, any motion or reaction could be run backwards and be just as valid as it is run forwards. It would break no laws other than the laws of thermodynamics for heat to flow from a cold body to a hot one; on a small scale nothing would necessarily be wrong with that. Having that understanding, I've always understood the laws of thermodynamics to be true only on a macroscopic scale. Given trillions of particles all interacting constantly, the laws of thermodynamics are generally going to be followed and on a large scale will always be true. My question is, is there some actual fundamental law requiring entropy to increase on any scale? If you get down small enough, isn't it possible for entropy to decrease in a reaction, or does that violate something other than mere probability?