Liquids in air take the form of a droplet due to a tension created on their surface called surface tension. At the interface between a liquid and a gas, the liquid surface behaves as it was a skin, or a membrane stretched over the fluid mass. For example, if you gently place a strip of paper on liquid surface, it will float as if being supported by the hypothetical surface. Similarly, when water drips, the hypothetical skin wraps around the liquid mass forming a droplet. Although such a skin in not actually present this conceptual analogy allows us to explain the commonly observed phenomenon : surface tension. These various types of surface phenomena are due to the unbalance cohesive forces acting on the liquid molecules at the fluid surface. Molecules in the interior of the fluid are surrounded by molecules that are attracted to each other equally. However, the molecules along the surface are subjected to a net force. The apparent physical consequence of this unbalanced forces along the surface is to create a hypothetical skin or membrane. The intensity of the molecular attraction per unit length along any line in the surface is called surface tension and is designated by Greek symbol sigma.