Invoking the theory of rubber elasticity, a polymer chain in a cross-linked network may be seen as an entropic spring. When the chain is stretched, the entropy is reduced by a large margin because there are fewer conformations available. As such there is a restoring force which causes the polymer chain to return to its equilibrium or unstretched state, such as a high entropy random coil configuration, once the external force is removed. This is the reason why rubber bands return to their original state. Two common models for rubber elasticity are the freely-jointed chain model and the worm-like chain model.