Surface Tension, the tendency for the surface of a liquid to behave like a stretched elastic membrane. Molecules within a liquid's interior are attracted equally in all directions because of cohesion, the force that holds molecules together. Surface molecules, however, are subject to cohesive force only from the interior of the liquid and from the sides, since there is no balancing pull from above the surface. As a result of this unequal attraction, which attempts to pull the surface inward, the entire surface acts as if it were under tension.

Small drops of liquid tend to take on a spherical shape because the surface tension acts to make each drop as small as possible. Capillary action, the tendency of liquids to rise within small-diameter tubes, is due in part to surface tension.

Water has a higher surface tension than most liquids; it is possible to float a steel needle or razor blade on water if the surface is not penetrated.