We have a general, vague idea of the difference. A solid is not easily deformed; whereas a fluid is easily deformed (we can readily move through air). Although quite descriptive, these casual observations of the differences between solids and fluids are not very satisfactory from a scientific or engineering point of view. A more specific distinction is based on bow materials deform under the action of an external load. A fluid is defined as a substance that deforms continuously when acted on by a shearing stress of any magnitude. A shearing stress (force per unit area) is created whenever a tangential force acts on a surface as shown by the figure in the margin. When common solids such as steel or other metals are acted on by a shearing stress, they will initially deform (usually a very small deformation), but they will not continuously deform (flows). However, common fluids such as water, oil and air satisfy the definition of a fluid-that is, they will flow when acted on by a shearing stress. Some materials, such as slurries,ƒ€š‚ tar, putty, toothpaste, and so on, are not easily classified since they will behave as a solid if the applied shearing stress is small, but if the stress exceeds some critical value, the substance will flow. The study of such materials is called Rheology and does not fall within the province of classical fluid mechanics. Although the molecular structure of fluids is important in distinguishing one fluid from another from another, because of the large number of molecules involved, it is not possible to study the behavior of individual molecules when trying to describe the behavior of fluids at rest or in motion. Rather, we characterize the behavior by considering the average, or macroscopic. Value of the quantity of interest, where the average is evaluated over a small volume containing a large number of molecules.