The molecules (or atoms or ions) of a liquid, like those of a solid (and unlike those of a gas), are quite close together; however, while molecules in a solid are held in fixed positions by intermolecular forces, molecules in a liquid have too much thermal energy to be bound by these forces and move about freely within the liquid, although they cannot escape the liquid easily. Although the molecules of a liquid have greater cohesion than those of a gas, it is not sufficient to prevent some of those at the free surface of the liquid from bounding off (see evaporation). On the other hand, the cohesive forces between the molecules at the surface of a mass of liquid and those within cause the free surface to act somewhat like a stretched elastic membrane; it tends to draw inward toward the center of the liquid mass, to draw the liquid into the shape of a sphere, thus exhibiting the phenomenon known as surface tension.
A liquid is said to "wet" a solid substance when the attractive force between the molecules of the liquid and those of the solid is great enough to hold the liquid's molecules at the solid surface. For example, water "wets" glass since its molecules cling to glass surfaces, whereas mercury does not since the adhesive force between its molecules and those of glass is not strong enough to hold them together. Capillarity is an example of surface tension and adhesion acting at the same time.