Heat cannot spontaneously flow from cold regions to hot regions.

The second law of thermodynamics perfectly fits our everyday experience: water never spontaneously transforms into ice. But... have you ever wondered why?

Ever since the discovery of atoms, we imagine gases as particles zipping around, and solids as particles arranged in a neat lattice. How likely is it, then, that the gazillions of atoms in water spontaneously arrange in an ice lattice? Practically impossible, without an external intervention. Nature only evolves towards the most probable states: that is the second law.