Elasticity is the ability of a material to return to its original shape and size after being stretched, compressed, twisted or bent. Elastic deformation (change of shape or size) lasts only as long as a deforming force is applied to the object, and disappears once the force is removed. Greater forces may cause permanent changes of shape or size, called plastic deformation.

In ordinary language, a substance is said to be "elastic" if it stretches easily. Therefore, rubber is considered a very elastic substance, and rubber bands are even called "elastics" by some people. Actually, however, most substances are somewhat elastic, including steel, glass, and other familiar materials.

The simplest description of elasticity is Hooke's Law, which states, "The stress is proportional to the strain." This relation was first expressed by the British scientist, Robert Hooke. He arrived at it through studies in which he placed weights on...