Imagine you’ve got a glass high up on a counter, and it falls down to the floor. Physics tells you what’s going to happen: your glass is going to hit the floor with a specific velocity, and with a certain calculable amount of kinetic energy. That impact can easily result in the glass shattering: a spontaneous process that results from the conversion of one form of energy into others. However, the reverse process — of shattered glass shards spontaneously reassembling and leaping the fully assembled glass back onto the counter — never spontaneously occurs. This can be simply explained by the laws of thermodynamics, and in particular by the second law.
A great many of us, particularly in the United States, learn about the second law in terms of entropy: a physical property of all thermodynamic systems. The second law can be expressed in terms of: