A spring is an elastic coil made of steel alloy that exerts resistance when it changes shape. The nature of the coil is such that it stores mechanical energy that can be applied to a number of sources, including machinery. Usually made from hardened steel or metal, the spring exerts energy when it changes shape and returns to its original shape when the force is removed. 
In the 17th century, Robert Hooke first penned a theory about coils – he noticed that when an elastic member experiences stress, it changes shape consistent to that stress. He also claimed that if the member extends past its elastic limit, it cannot return to its former shape. 
Springs were used for centuries before the existence of Hooke's theory. During the third century B.C., a Greek engineer named Ctesibius developed a springy bronze made from tin and copper alloy. After casting it and hammering it to produce a hardened device, he added leaf springs to utilize it for a military catapult but it proved unsuccessful.
By the second century B.C., another engineer, Philo Byzantium, built a similar device and this time it was successful. Springs continued to be used for different purposes and developed extensively from one century to the next. Springs replaced weights in clocks and the onset of the Industrial Revolution saw British locksmith Joseph Bramah developing a spring winding machine in his factory. After this development in the 1780s, springs became more widely used.
Springs typically take a cylindrical or conical shape and exist in different types. The most common of these include extension, compression, and torsion. Extension springs are the type of coiled springs in which the springs touch each other and separate when force is exerted; compression springs have spaces between the coils that join when force is exerted; finally, the torsion spring is made up of coils that wind into a tighter spiral when force is applied. Torsion springs are commonly found clipboards and butterfly hairclips.
Another variation is the watch spring, which is a flat spiral rather than a cone or cylinder. Some springs do not have coils, such as the leaf spring. The leaf spring, with its shallow arch shape, is utilized in automobile suspension systems. 
Today, springs are used to make an unlimited amount of objects and machinery, from key chains, to cellular phones, to catheters. Springs can be found in almost every machine and mechanical device.