Osmosis

Osmosis is the diffusion of liquids in one direction through a semipermeable membrane—that is, one which will allow small molecules through but not large ones. An example is a container of sugar solution divided in two by a sheet of cellophane. On one side the solution is strong, on the other it is weak. Water can get through the cellophane but sugar cannot. The result is that water flows through the membrane from the weak side to the strong side. This movement can generate considerable pressure, which is known as osmotic pressure.