Gases and liquids are considered fluids because they both flow and move. The molecules comprising both gases and fluids are constantly in motion in random formations. The molecules continuously collide with each other and the sides of any container in which they are placed.
The four phases of matter are solids, liquids, gases and plasmas. The term "fluids" is typically used to refer to liquids in everyday life as well as in both medical and nutritional terms; gases are considered to be fluids in terms of physics. Other substances are difficult to classify as true fluids. These are typically referred to as viscoelastic fluids. One example of a viscoelastic fluid is Silly Putty, because it has characteristics of both a solid and a liquid based on its viscosity at the time it is observed.
The molecules of substances that are in a liquid phase are weaker than those in a solid phase. Liquids always take the shape of the container that holds them, but they do not increase in volume to fill the container. The molecules of gases are very weak when compared to those of solids. Gases always take on both the shape and volume of the container that holds them.