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.