Liquid is a state of matter in which the constituent molecules are not constrained in their relative location to each other. In the way it flows, a liquid behaves similarly to a gas. Unlike a gas it occupies a nearly fixed volume, regardless of the pressure. Where in contact with a gas, a liquid will be bounded by a well-defined surface, whose strength is described by a property called surface tension. Surface tension is responsible for most of the phenomena we associate with liquids, including wetting to surfaces, and the tendency of small droplets to form spheres.