So the entropy of the universe must always increase. And that's equivalent to saying the total amount of energy in the universe after the reaction must be more spread out than before that reaction actually took place. Now, we can also describe the second law of thermodynamics by using a mathematical equation. So we can say that the change in entropy of the system plus the change in entropy of the surroundings is equal to the change in entropy of the universe. And that must always be greater than zero. It must always increase.