Solids retain their shape because strong forces hold the atoms in their places. In a liquid the forces keep the atoms close together, but they are free to move. In a gas the atoms are about ten times further apart than in either solids or liquids and forces between them are very weak. As a result of weaker forces, a liquid or gas can flow freely and assume the shape of their container. They are called fluids.

There are two main areas of fluid study. The field that studies fluids in a state of rest is called static fluids, and the field that analyzes the movement of fluids is called fluid dynamics. We will first explore static fluids.