When a spring or a rubber band is stretched and released, they come back to their original size & shape.This physical property of a material called ‘elasticity’. When subjected to external forces, a solid body may deform. Then internal forces come into play and try to restore the body in its original shape. When the deformation is very large, the body may be permanently deformed; for example, if we try to stretch the rubber band to a very large extent, then it breaks after a certain point and it gets permanently deformed. But in case of a spring the same magnitude of force may not cause a permanent deformation. The breaking force can vary from material to material. Similarly, the extent to which the shape of a body is restored when the deforming forces are removed, varies from material to material. The property of the material that restores the natural shape or resists the deformation is called the elasticity of a material. As a measure of the stiffness of a material, the knowledge of elasticity of materials used in construction is important in engineering designs.