A spring pushes back in proportion to how far you stretch or squeeze it: F = k·x, where k (N/m) is its stiffness. Hang a mass on it and it stretches until k·x = mg. Every solid behaves like this for small deformations, which is the basis of all structural engineering. Steel is just a very stiff spring. Past the elastic limit the line bends over, and the material stays permanently deformed.