The usual definition of a fluid is a substance that can sustain no static shear stress. Thus a fluid takes the shape of its container, but a shear stress will develop if the fluid flows. We know that some fluids flow more readily than others and a quantitative property of a fluid that describes the ease of flow is the viscosity. Let us put a thin layer of a fluid between two parallel plates, move one plate at a velocity U, and keep the other plated fixed. The width of the plates are to be large compared to the space between them. The force to move the plate divided by the area is the shear stress. The viscosity of the fluid is the ratio of this force to the shear rate (the shear rate here is the velocity divided by the plate spacing). Fluids with a low viscosity require less force to keep the plate in motion at the constant speed U than fluids with a high viscosity. This experiment can be carried out at many different speeds or plate spacings so that a complete curve of shear rate vs shear stress can be found. If this curve is a straight line which passes through the origin, the fluid is called Newtonian. Otherwise we say the fluid is non-Newtonian.