Instability is the condition of a system when it is easily disturbed by internal or external forces or events, in contrast to a stable system, which will return to its previous condition when disturbed. A pencil resting vertically on its eraser or a coin standing on edge are examples of systems that have the property of instability since they easily fall over at the slightest breeze or movement of the surface they are resting on. An unstable system is one whose attractors can change; thus, instability is a characteristic of a system that is far from equilibrium or at bifurcation.