To answer this question it is first necessary to understand what Osmosis is.
Osmosis is a natural process that occurs in all living cells. Water permeates through a membrane that excludes suspended solids, dissolved salts and larger organic molecules. These semipermeable membranes have pores of approximately 0.0005 microns in size.
Water molecules have a stronger tendency to escape from pure water than from a salt solution. Water flows through the semipermeable membrane from the pure solution to the salt solution in an effort to equalise the osmotic pressure of the two solutions.