Adding a crystal of a solid to a solution super-saturated with that solid

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.

I actually had some advanced chemistry students do this in chemistry lab.

I was teaching a physical chemistry lab, and the students were trying to measure the enthalpy of solution of oxalic acid in water. They prepared a saturated solution at high temperature, and allowed samples to cool down to various temperatures. Some of the oxalic acid was supposed to precipitate, but in most cases, nothing precipitated immediately, and I went through all the usual tricks of the trade, such as agitating the solution, scratching the glass, etc. Finally I told them to get the bottle of oxalic acid and drop a few crystals in each test tube.

Instant success. Immediately crystals began to form. Not just tiny little things that made the water cloudy, but needle-shaped things several mm long. They continued growing over the next few seconds, and soon the test tubes were full of huge needle-shaped crystals. The expressions on the students' faces were priceless, as if they'd never expected anything a child might do at home with sugar or salt would ever get them through a 300-level college chemistry lab.