In a liquid, the particles are fairly close together. They are free to change places. There are some forces between the particles. These forces keep most of the particles together in the liquid. As the liquid is heated, the particles move faster and faster. Eventually they move so fast that they break free from each other. When this happens, we say the liquid has boiled. The vapour that is formed is made from the liquid particles flying around at high speeds in all directions. This is shown in the second diagram. The particles are the same as they were in the liquid, they are no bigger or smaller. It is what they are doing that changes.