Water and all other liquids resemble solids in that they possess a definite size; that is, they occupy a definite space. Liquids differ in that they have no definite shape. The shape of a liquid is the shape of the vessel which holds it. A solid has a definite shape and retains it until acted upon by a force greater than the cohesive strength of its particles. The force of gravity is continually forcing liquids to seek the lowest level. This fact is illustrated when two vessels containing the same liquid are connected. The level in each becomes the same, regardless of the form or distance of the connecting pipe. This peculiarity of liquids is commonly expressed by the saying "water seeks its own level."

A force of any kind, however small, will change the shape of a liquid. To illustrate: If a pebble is dropped into a pond it moves the whole of the water and the motion can be seen by the ripples which form on the surface of the pond. The rate of this change in shape varies with different liquids. Those in which the change proceeds slowly are called viscous liquids, while liquids in which the change takes place quickly are called mobile.

Another important property of liquids is that they cannot be compressed. If force acts on any part of a liquid, it will transmit the pressure of the force equally in all directions. This principle, which is called Pascal's law from its discoverer, renders liquids very valuable as a medium for pressure transmission in all forms of hydraulic machines.