A liquid is a substance that flows more freely than a solid, but not as freely as gas. When a substance is liquid, its atoms or molecules (groups of atoms) possess more energy than they have when it is solid, but less energy than they have when it is a gas. This allows liquid to flow and take the shape of its container and form a level surface. Liquids are much harder to compress than gases. This why they are often used to transmit forces in hydraulic machines.