Capillary Action

The movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension. Surface tension is a measure of the strength of the water's surface film. The attraction between the water molecules creates a strong film, which among other common liquids is only surpassed by that of mercury. This surface tension permits water to hold up substances heavier and denser than itself. A steel needle carefully placed on the surface of a glass of water will float. Some aquatic insects such as the water strider rely on surface tension to walk on water.

Capillary action occurs because water is sticky, thanks to the forces of cohesion (water molecules like to stay closely together) and adhesion (water molecules are attracted and stick to other substances). So, water tends to stick togther, as in a drop, and it sticks to glass, cloth, organic tissues, and soil. Dip a paper towel into a glass of water and the water will "climb" onto the paper towel. In fact, it will keep going up the towel until the pull of gravity is too much for it to overcome.

This is more important than you think. Consider:

One common experiment to demonstrate capillary action is to place a stalk of celery in a glass of water that has been colored with food coloring (you might want to use a piece of celery that has begun to whither, as it is in need of a quick drink). This effect happens because, in plants, water molecules move through narrow tubes that are called capillaries.

Definition from the USGS, URL: