Janet Huffstetler, age 15, of Wilmington, Del., for her question:

JUST WHAT IS ELASTICITY?

Elasticity is the property of a material that causes it to resume its original size and shape after having been compressed or stretched by an external force. In science and in common language, terms such as stiffness, tensile strength and resilience are often used to describe elasticity.

The applied force creates stress within the material, expressed as units of force per unit area, and causes a deformation called strain.

Stress and strain are determined by the molecular structure of the material. The distance between molecules in a stress free material depends on a balance between molecular forces of attraction and repulsion.

When an external force is applied, the molecular distances change and the material becomes deformed. When the force is removed, the molecules return to their balanced position, and the elastic material regains its original shape.

The various types of stress strain deformation are called the elastic moduli of a material.