Viscous materials, like water, resist shear flow and strain linearly with time when a stress is applied. Elastic strings strain when stretched and immediately return to their original state once the stress is removed.

Elasticity is usually the result of bond stretching along crystallographic planes in an ordered solid, where as viscosity is the result of the diffusion of atoms or molecules inside an amorphous material.

The graphs in Figure 16, compares strains on ideal viscous liquids, and ideal elstic solids when a constant stress is applied on them for given time t.