Thus, a fluid can be defined unambiguously as a material that deforms continuously
and permanently under the application of a shearing stress, no matter how small.
This
definition does not address the issue of how fast the deformation occurs and as we shall
see later this rate is dependent on many factors including the properties of the fluid
itself. The inability of fluids to resist shearing stress gives them their characteristic
ability to change shape or to flow; their inability to support tension stress is
an engineering assumption, but it is a well-justified assumption because such stresses,
which depend on intermolecular cohesion, are usually extremely small.....
Because fluids cannot support shearing stresses, it does not follow that such
stresses are nonexistent in fluids. During the flow of real fluids, the shearing
stresses assume an important role, and their prediction is a vital part of engineering
work. Without flow, however, shearing stresses cannot exist, and compression stress or
pressure is the only stress to be considered (Elementary Fluid
Mechanics, 7th edition, by R.L. Street, G.Z. Watters and J.K. Vennard, John Wiley \&
Sons, 1996).
So we see that the most obvious property of fluids, their ability to flow and
change their shape, is precisely a result of their inability to support shearing stresses.
Flow is possible without a shear stress, since differences in pressure will cause a fluid
lump to experience a resultant force and produce an acceleration, but when a fluid is
deforming its shape, shearing stresses must be present.
With this definition of a fluid, we can recognize that certain materials that look like solids are actually fluids. Tar, for example, is sold in barrel-sized chunks which appear at first sight to be the solid phase of the liquid which forms when the tar is heated. However, cold tar is also a fluid. If a brick is placed on top of an open barrel of tar, we will see it very slowly settle into the tar. It will continue to settle as time goes by --- the tar continues to deform under the applied load --- and eventually the brick will be engulfed by the tar. Even then it will continue to move downwards until it reaches the bottom of the barrel. Glass is another substance that appears to be solid, but is actually a fluid. The glass flows under the action of its own weight. If you measure the thickness of a very old glass pane you would find it to be larger at the bottom than at the top of the pane. This deformation happens very slowly because the glass has a very high viscosity, and the results can take centuries to become obvious. Another example: silly putty behaves like an elastic body when subject to rapid stress (it bounces like a ball) but it has fluid behavior under a slowly acting stress (it flows like a fluid under its own weight).