 Now, we have seen right from our introductory video that thermodynamics studies interactions between systems, notice the plural systems. And we have said the interactions are energy interactions. We will again come to interactions later, but let us say that interactions is something like a transaction, just the way we have, we go to a bank and withdraw money from our account that is a financial transaction, it is a monetary transaction, it is a give and take, something comes out of the account into your hand or if you deposit money, something from your pocket goes into your bank in your account. So interaction is like a transaction, that was a monetary interaction, here we will be talking about energy interactions. And because interactions involves at least two systems, interaction is always between two systems and is across the boundary separating them. Hence it is necessary for us to be clear about the idea of the two interacting systems and the boundary separating them. A general model could be something like this, we have one system, we have another system nearby and we have a boundary separating them, a common boundary. So we can say this is the system which is under study, let us call it system A, this is the second system with which the first system system A is interacting, let us call it system B and this is the boundary separating A and B, system A and system B and interactions of energy take place between the two systems across this boundary. In fact system A will be a proper thermodynamic system, system B will also be a proper thermodynamic. However, quite often one of the two systems says system A is the system of primary interest, in which case we may simply call it the system, this could be then the secondary system, the primary system and the secondary system. So we may call the two systems as system A or system B or system of primary interest or simply the system, system B could be the secondary system but quite often the secondary system is given the name surroundings. Remember that although quite often we will be talking of the system and its surroundings, remember that the surroundings is also a thermodynamic system in its own right, it should also be a properly defined thermodynamic system, it should also have all its boundaries properly defined. However, quite often we come across situations where our primary system is of importance to us, we call it the system, whereas the real surroundings, say suppose I am the system, the surrounding is this room, in particular the air in this room. So that air I can consider it a large system and during my interaction with the air in the few minutes that I am going to sit here and talk and make some gestures and write something, the situation in that air is not going to significantly change, it will there will be some effect but that it will be a minor effect, such a surrounding system is quite often known as an environment, these are just the different words using the textbooks of thermodynamics and when we discuss thermodynamics but we should remember that the surroundings of the environment is as much a properly defined thermodynamic system as our system itself or the primary system itself, we can stop here.