Substances in their gaseous or liquid phases are considered fluids. Fluids are held together by their own cohesive forces and take the shape of their container. Fluids are also characterized by their ability to flow when under the influence of even small forces. The flow velocity along with its density and pressure are important quantities in describing the state of a given fluid volume at a given time.
Both fluids and solids can resist forces exerted in a direction perpendicular to the surface. A fluid submitted to a tangential force, however, will move until the forces exerted are entirely perpendicular. The tangential force causes the layers of the fluid to slide along each other until the tangential force is no longer present. Fluids are either at rest (static) or in motion (dynamic).
