ABSTRACT
Intuitively, a system is in a stable state if it can recover from perturbations; a small change will not prevent the system from returning to equilibrium. Place a marble at the center of a smooth bowl and tap it slighty. After some rolling back and forth, the marble will return to its stable position. If instead you turned the bowl over and put the marble carefully on top at the center, then it would be in a rather unstable equilibrium; a slight breeze would suffice to make it fall.
