When two different liquids or liquids with a different concentration are separated by a permeable membrane, nature tries to establish a balance between both liquids or concentrations, depending on the permeability/"mesh size" of the membrane, the kind of liquids and the pressures applied. If there is the same pressure on both sides of the membrane and the medium is water, for example, then the water containing more minerals will try to dilute with the less concentrated water through the membrane. This process is called osmosis. If the pressure on the side with the more concentrated water is higher than on the side with less concentrated water, the water will flow in the opposite direction. This process is called reverse osmosis.