The growth of crystals
Try to imagine the natural formation of crystals in the earth. In the earth’s surface there is a big mixture of all kinds of atoms. They are being continually churned about by volcanic action, by wind, and by water—continually being moved about and mixed. Yet, by some trick, silicon atoms gradually begin to find each other, and to find oxygen atoms, to make silica. One atom at a time is added to the others to build up a crystal—the mixture gets unmixed. And somewhere nearby, sodium and chlorine atoms are finding each other and building up a crystal of salt.  How does it happen that once a crystal is started, it permits only a particular kind of atom to join on? It happens because the whole system is working toward the lowest possible energy. A growing crystal will accept a new atom if it is going to make the energy as low as possible. But how does it know that a silicon—or an oxygen—atom at some particular spot is going to result in the lowest possible energy?  It does it by trial and error. In the liquid, all of the atoms are in perpetual motion. Each atom bounces against its neighbors about $10^{13}$ times every second. If it hits against the right spot of growing crystal, it has a somewhat smaller chance of jumping off again if the energy is low. By continually testing over periods of millions of years at a rate of $10^{13}$ tests per second, the atoms gradually build up at the places where they find their lowest energy. Eventually they grow into big crystals. 