Osmosis

When water diffuses through a semipermeable membrane from an area of high concentration to an area of low concentration this process is called osmosis. The small water molecules are black and the larger molecules in blue represent a different substance, such as salt.

See how the water molecules are able to pass through the membrane, increasing their concentration on the right side of the membrane. The number of water molecules on the right will continuue to increase until the concentration of water molecules is about the same on both sides of the membrane. At this point as many water particles cross in either direction and the system has reached the equilibrium concentration.

The cell membranes of all living organisms are semipermeable membranes. So much water enters a red blood cell by osmosis when it is placed in fresh water that it bursts!

Osmosis is the reason that the skin on our hands wrinkles into "prune fingers" when we stay too long in the bath or shower. Water diffuses from the higher water concentration (but low salt concentration) of the fresh water in the bath into the lower water concentration (but higher salt concentration) of our skin, causing it to swell and be thrown into wrinkles.

The exact opposite happens when we immerse ourselves in sea water at the beach, particularly on our hands and faces, and our skin shrinks by osmosis and feels tight.