When a crystal of a solid is added to a super-saturated solution of that solid, there is a rapid precipitation of the solid within the solution.
The best way to observe this impressive phenomenon:
1. Heat
water to a very high temperature.
2. Add sugar until you cannot
dissolve any more; remove any
excess sugar.
3. Very slowly cool the water back down to room
temperature. The
solution is now super-saturated.
4. Add a crysal of
sugar to the
solution.
5. Watch the magic of rapid
mass precipitation.