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. Now, the next question is what exactly do we mean by entropy? Well, let's look at diagram A and diagram B. In diagram A, we have a situation in which this container is our system, and the molecules inside are also part of our system.