So the second law of thermodynamic states that the entropy of the universe always increases. Now, that doesn't mean that the change in entropy of a system cannot be negative. So the entropy of a system can still decrease as long as the entropy of the surroundings increases by greater amount, so that this is always true. So the entropy of a system can decrease, can be negative, as long as the entropy of the surroundings is increased by a greater amount. So that when we take the sum, we see that the change in entropy of the universe is always equal to a positive value. It is never equal to a negative value.