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A metal undergoes plastic flow as atoms change neighbors. A liquid undergoes viscous flow as molecules change neighbors. But the plastic flow of the metal differs from the viscous flow of the liquid in an important way. When a metal undergoes plastic flow, the stress depends on the amount of deformation, but is insensitive to the rate of deformation. By contrast, when a liquid undergoes viscous flow, the stress is insensitive to the amount of deformation, but depends on the rate of deformation. In studying plasticity, it is useful to recall the law of viscosity.