A liquid is a state of matter characterized by flowing properties and the ability to conform to the shape of its container. Unlike gases, liquids maintain a fixed volume, and unlike solids, they exhibit fluidity, meaning their constituent particles can move past one another with minimal resistance. The intermolecular forces within a liquid are sufficient to hold the substance together while remaining weak enough to allow for continuous flow and surface adaptation. Key physical properties include density, viscosity, and surface tension, which quantify the liquid's resistance to flow and its tendency to minimize surface area. The changes between solid, liquid, and gaseous states are defined by temperature and pressure, requiring the input or release of energy to transition between phases.