 There are three main states of matter. First of all, what is matter? Matter is anything that has mass and occupies space. So two requirements for being matter. The three main states are solid, liquid and gas. Now when something is a solid, the shape of the matter does not depend on the container. It is fixed. Also, the volume is fixed. I could put this little cube of solid matter in a bigger container. It would have exactly the same shape and the same volume. If we would be able to zoom in a lot, we would see that the individual molecules of something that is solid are moving slightly. But overall, they're stuck together. There's a lot of interaction between them, keeping them kind of in a grid. When something is a liquid, then the shape is not fixed anymore. A liquid will always take the shape of the container it is in. Like the water, for example, will take the shape of the glass at the bottom once it is liquid. The volume, however, is fixed. If I put 5 milliliters of water in a huge container, it will still be just 5 milliliters of water. If you look at a molecular level, now the movements of the molecule is going to be much bigger and the interactions are much smaller than in a solid. However, there are still some interactions between the molecules, giving them not so much freedom, as in the last step, which is gas, where basically there's no more interaction between the molecules. There is a huge space in between them and they're flying around quite freely and will occupy whatever volume is given to them. So if something is a gas, then the shape is not fixed. It will take whatever space you give it. And also the volume is not fixed. When matter changes from one state to another, it undergoes a state change. Many of the names for those changes you already know. For example, if a solid becomes liquid, you call this melting. If then the liquid becomes a gas, that's either boiling or evaporation. The other way around, if a gas becomes liquid, for example in your shower, if you take a shower in the winter and then suddenly there's like a cover of water on your mirror, that's called condensation. And if the liquid becomes solid, this is freezing or solidification. It becomes solid. But did you know that a gas can directly become solid without passing through the liquid state? For example, at the entrance of a freezer, if there's some humid air that comes in contact with a very cold surface, the water in the air can directly form a solid, which you can see as this little white ice around the top of the freezer. This process is called a position. If you take me around, you can go directly from solid to gas without passing through the liquid phase. If I put a cube of ice in the freezer and I wait a very, very long time, the cube of ice will be gone. This way around, we call it sublimation. Now what's interesting is that all the changes that go in that direction, so from solid to liquid to gas or from solid directly to gas, they will require energy. If you want to melt the solid down, you need to add energy. You need to put it on a hot surface. If you want to go from liquid to gas, same thing. If you're boiling water, you need to add energy to make the liquid water into vapor. Now the other way around, it will actually release energy. So if some gas is condensating, it will actually warm up the surface on which it is condensating. If a liquid is freezing, the moment it is freezing, it releases a little bit of energy. If I will be able to measure the temperature of ice that becomes liquid water and then gas, and that will be starting from ice, I would start from a very cold temperature, below zero, otherwise it wouldn't be ice. And the more energy I add, remember I need to add energy, the warmer it gets. The moment I hit the melting point, something interesting is going to happen. Some of the molecules are going to stay changed and they're going to become liquid. This needs a lot of energy, so instead of warming up the ice more, my entire block of ice will actually stay at around zero degrees Celsius until all the water is melted. Once all the water is melted, all the water is going to get warmer as long as I keep adding energy, until some of the water starts evaporating. At this point, the energy that goes that I add is used to evaporate the water and my temperature of the entire water stays the same. If you boil in water, the temperature of the boiling water will never exceed the boiling point until all the water is completely evaporated and then the energy writing is going to be used to increase the temperature of the gas or the vapor in the case of water. If you will do the other way around, if you start with some vapor and you cool it down by removing energy, you cool it down, first of all, the vapor gets colder and colder and colder until some of it becomes liquid and only once all of the vapor is liquid, then the liquid will start getting colder and colder until you hit the freezing point and only once the entire amount of liquid is frozen then will the ice get colder and go below zero. This, for example, makes that the lake in the winter will not necessarily completely freeze because the water in the lake will stay around zero until the entire lake is frozen. Any more energy that's removed with the cold air that's on top will first be used to form more ice and by forming more ice there's a tiny amount of energy released which keeps the water below the ice above the freezing point until literally everything is frozen.