Much of what we value in life is fluid: a breath of fresh winter air; the hot blue flame in our gas cooker; the water we drink, swim in, and bathe in; the blood in our veins. So, what exactly is a fluid?

Matter most commonly exists as a solid, liquid, or gas; these states are known as the three common phases of matter. Solids have a definite shape and a specific volume, liquids have a definite volume but their shape changes depending on the container in which they are held, and gases have neither a definite shape nor a specific volume as their molecules move to fill the container in which they are held. Liquids and gases are considered to be fluids because they yield to shearing forces, whereas solids resist them. A container, for example, is a shearing force, since it forces liquids and gases to change shape based on its dimensions. Solids don’t change based on their container.

Atoms in solids are in close contact, with forces between them that allow the atoms to vibrate but not to change positions with neighboring atoms. Thus a solid resists all types of stress.

In contrast, liquids change easily when stressed and do not spring back to their original shape once the force is removed because the atoms are free to slide about and change neighbors—that is, they flow with the molecules held together by their mutual attraction.

Atoms in gases are separated by distances that are large compared with the size of the atoms. The forces between gas atoms are therefore very weak, except when the atoms collide with one another. Gases thus not only flow but they are relatively easy to compress because there is much space and little force between atoms.