the solution becomes supersaturated.
This means that the solution has reached a
delicate balance of just enough sugar molecules and
enough heat to keep the sugar molecules dissolved,
but in an unstable state. The sugar molecules begin to
crystallize back into a solid at the least disruption of
heat or action. In other words, the sugar comes back
together as sugar crystals when the syrup cools.
To make candy, you boil a mixture of sugar and
water to create sugar syrup. The water evaporates,
and the sugar concentrates. The higher the temperature,
the more concentrated the sugar becomes. The
texture of a candy (hard, soft, or chewy) depends on
its cooking temperature and ingredients.
References
http://www.exploratorium.edu/cooking/candy/index.html
http://en.wikipedia.org/wiki/Candy