Surface tension

Surface tension, property that makes the surface of a liquid act as if it were an elastic film. The molecules in a liquid stay together because they are pulled towards each other equally. This force is called cohesion. The molecules at the surface of a liquid have no molecules above them to attract and pull them; these molecules are pulled only by those beside and below them. This continuous sideward and downward pull creates tension at the surface, making it act as if it were a resilient film. A razor blade placed flatly on the surface of water does not sink because the surface tension supports it. Because of surface tension some birds are able to stand on water.

See also: Capillarity; Cohesion.