In the present context of fluid mechanics, it is important to distinguish the behavior of a fluid from that of a solid when subjected to a shear force (parallel to its surface).

Consider a solid element ‘ABCD' shown below (Fig 1a). The bottom section A B is fixed. A small horizontal force is applied on the surface CD. The surface is ‘sheared'. The element CD deforms into C'D' and then, into C”D”. However, an equilibrium position is achieved when the deformation stops. The top layer is now stationary. The solid has, therefore, resisted the applied shear force.