Entropy, abbreviated with the letter S, is a fundamental physical quantity in thermodynamics. To understand entropy, we consider a closed system. A closed system is an isolated part of the universe that does not interact with its surroundings. Let us consider the following system, which consists of subsystem A and subsystem B, in which four particles can reside. Another concept that is necessary for understanding entropy is the micro-state of a system. A micro-state of the system is characterized by how the four particles are distributed between the two subsystems. Let's take a small example. There are two particles in subsystem A. The other two particles are in subsystem B. This is one possible micro-state of the system. Another possible micro-state is when all particles are in subsystem A and none at all in subsystem B. A micro-state is therefore a possible arrangement of the particles. Let us denote the number of possible micro-states of a system by N. Now we can understand entropy. Entropy is a measure of the number of possible micro-states. The more states the system can occupy, the higher its entropy. Let's construct a system in which no particles can be in subsystem B. All four particles must be in subsystem A. In order to be able to make a statement about the entropy of this system, we must consider the number of possible states. The system was constructed in such a way that all particles can only be in A. There is therefore only one micro-state of the system, namely the state, all particles in A. Let's look at another system in which there is more than a single micro-state. Let us now assume that there can be a maximum of two particles in subsystem B. The first possible micro-state is where all particles are in subsystem A. The second allowed micro-state is where a single particle is in subsystem B. And the last permitted micro-state is where two particles are in subsystem B. Placing another particle in subsystem B is not allowed in the system. This system has a higher entropy than the previously considered system, where particles could only be in subsystem A. Let us now consider a third system. Let us assume for this system that there are no restrictions on how particles can be distributed in subsystems A and B. Let's go through all possible micro-states of the system. In the first micro-state, all particles are in subsystem A. The second allowed micro-state is where a single particle is in subsystem B. The third permitted micro-state is where two particles are in subsystem B. The next permitted micro-state is where there are three particles in subsystem B. And the last allowed micro-state is where all four particles are in subsystem B. This system has five possible micro-states. With only four particles, a maximum of five micro-states can be realized. Which of the three systems has the maximum and minimum entropy? The first system has a single micro-state. This system has the lowest entropy S1. The second system has three possible micro-states, and therefore has a higher entropy S2. And the third system has five possible micro-states. The third system has the highest entropy S3.