A spring is an elastic object that stores mechanical energy. Springs are typically made of spring steel. There are many spring designs. In everyday use, the term often refers to coil springs. Most extension springs are wound with initial tension. This is an internal force that holds the coils tightly together. The measure of the initial force and just start coil separation. Unlike a compression spring, which has zero loads at zero deflection, an extension spring can have a prelude at zero deflection.