Fluid mechanics is the study of liquids and gases that are either at rest or in motion. It is present in our everyday lives either directly or indirectly, and the first thing that you need ask yourself is what a fluid is? Scientifically speaking a fluid is based off of how a substance’s molecules behave. A fluid’s molecules are allowed to move freely, instead of being held together like they would be for a solid. Basically what this means is any substance that will readily deform under any magnitude of shear stress is considered a fluid.
There are two different states that a fluid can take. These states are a liquid or a gas. Both will act like a fluid, however there are couple of key differences between a liquid and a gas. First, a gas will expand to fill the volume of its container, a liquid will not. Finally, a gas compresses easily, while a liquid will not, and in most cases we can assume that liquids are incompressible due to the magnitude of pressure required to compress them.
Finally, there are substances that have characteristics of a fluid and a solid. An example would be tooth paste. Notice that tooth paste will hold its shape, but when a shear stress is applied to it, it will deform. The name of the field that studies these types of substances is rheology.