Osmosis is the diffusion of water across a semipermeable membrane. Osmosis is a passive transport mechanism; therefore, it does not use any energy in order to make it happen. It works based on concentration differences. Let's use a simple example to illustrate. If the inside of a cell is 80% water and 20% salt, and the area outside the cell is 60% water and 40% salt, there is a greater concentration of water inside the cell than outside the cell. This means that inside the cell is a hypotonic solution (less solute/salt), and outside the cell is a hypertonic solution (more solute/salt). Water "wants" to seek equilibrium and have equal concentrations inside the cell and outside the cell; therefore, the water inside the cell will diffuse through the cell's membrane to the outside environment until equilibrium has been reached. Since water is leaving the cell, the cell will shrink in this example.