When the liquid in a limited evaporation in a confined space, through the surface of the liquid into the space above, a steam elements. As steam heat elements in the movement disorder, collide with one another and, as well as surface and the vessel wall collisions, and the surface in the collision, some elements have been attracted by elements of liquid, and return to the liquid in a liquid elements . At the beginning of evaporation, access to space more than the number of elements to return to the number of molecules in the liquid, along with the continued evaporation, steam room of the increasing density and thus return to the number of molecules in the liquid also increased. When the unit time access to space and the number of molecules in the liquid to return to the same number of elements, the evaporation and condensation in a state of balance, although at this time of evaporation and condensation is still in progress, but in the steam room will not increase the density of molecules, At this point the state known as the saturation point. The saturation point under the liquid known as saturated liquid, known as the dry steam saturated steam (also known as steam).