where p is a parameter or input. Suppose that when p=0 the function has three roots, x1 < x2 < x3. We assume that x1,x3 are stable roots and x2 is unstable. If initial conditions are started below x2 then the system will tend to the lower root x1 whereas if the initial conditions are greater than x2 the solution will tend to x3 . The key point is that there are two distinct stable states. If there is a way to transiently kick x above the threshold , x2 then x will stay at the higher state until something kicks it down again. The system now has a "memory" and can hold onto that memory long after the stimulus or kick goes away.