So, this is a broad introduction to cavitation, why it is important. There are number of effects of cavitation, we will see them slowly during the course of our lecture. Now, cavitation you can understand why cavitation occurs in the first place. If you have a water mass somewhere open to atmosphere at normal temperature, always there will be some water evaporating the atmosphere and some water coming back to the condensing onto the surface of water. So, when the water drops get into water they exert a small pressure on the water because it becomes heavier than air and falls into water therefore, there is a small pressure on the water. Depending on the temperature, that pressure that is that is exerted by incoming water drops at the surface water is generally called the vapor pressure. That is at that temperature this is the pressure exerted by water vapor on the surface of water on the water surface.