 Welcome back. Now let us look at the basic difference from a thermodynamic point of view between various simple compressible systems. Let us look at gaseous systems, liquid systems and solid systems. Here I have a series. We have seen this earlier in some illustration. It is generally an open system. There is a spout here through which fluid can come in or go out. But if I hold tight here, it is a closed system. Try to push out gas, but it will not go out. Now let us see. There is about 50 cc of air in it. It is a gas. If I close it and try to compress it, I can compress it, may be roughly to half its volume without much of an effect. So remember that gas as a simple compressible system is reasonably easily compressible. Now I am going to have some liquid water in this. Now see, I have filled it up with water and I have attempted to remove all air bubbles from it. Now let me try to compress it. Notice that in spite of great effort, I am just not able to compress it, not even by one cc, a fraction of a cc in 50 cc. So this indicates that although liquid is also a fluid, simple compressible system containing nothing but a liquid, it is almost incompressible. Of course I am not a very strong person, but if you really get a strong person, you may be able to compress it, but a very small bit. Now let us look at a simple solid. I have here a solid piece or even this computer mouse is essentially a solid. And what do I do if I try to compress it? Nothing will happen. In fact if I try to compress it too much, I am able to hurt myself. In fact as I try to compress it, my fingers get compressed because my fingers are not a solid system. They contain some fluid inside. So remember that from a thermodynamic point of view, purely a thermodynamic point of view, the difference between a solid, a liquid and a gas is essentially our ability to compress them. Thank you.