Elasticity is the ability of a solid material to return to its original shape after it has been misshapen.
A force that misshapes material is called a stress.
All solids are slightly elastic but some are very elastic, for example rubber, thin steel and young skin.
Solids return to their original shape after the stress stops, as long as the stress is less than their elastic limit.
Strain is how much a solid is stretched or squeezed when under stress, namely how much longer it grows.
The amount a solid stretches under a force — the ratio of stress to strain — is called the elastic modulus, or Young’s modulus.
A bungee jumper stretches a piece of elasticized rope to a great extent. The rope then returns to its original length pulling the jumper back in the air. The greater the stress, the greater the strain. This is called Hooke’s law, after Robert Hooke (1635-1703).
Solids with a low elastic modulus, such as rubber, are stretchier than ones with a high modulus, such as steel.
Steel can be only stretched by 1% before it reaches its elastic limit. If the steel is coiled into a spring, this 1% can allow a huge amount of stretching and squeezing.
Some types of rubber can be stretched 1000 times beyond its original length before it reaches its elastic limit.