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Why does the sugar clump onto the string?

Dissolving the sugar in the hot solution (water) and then allowing it to cool creates what's called a supersaturated solution — that is, one that can't absorb any more sugar at that temperature. When this happens, the solution is unstable, meaning that it has more sugar in it than can actually stay in the liquid.

So molecule by molecule, sugar crystals in the solution will attach themselves to the sugary string — the crystals you put on the string when you soaked it essentially serve as starting points and are known as seed crystals.

Also, as the time goes by, the water evaporates very slowly from the solution , so sugar molecules continue to come out of the remaining solution and move onto the seed crystals on the string. Sugar molecules have a particular shape; they don't, for example, look like snow or diamond crystals. After millions of them layer onto the string, their shape will take a certain form. Even with the sugar growing at the rate of millions of molecules per hour, it'll take a day for your crystals to be visible and a week for them to use up all the sugar they can from the solution. In the end, the finished piece of rock candy will be made of about a quadrillion (1,000,000,000,000,000) molecules attached to the string.

Thanks to Don Rathjen, physics teacher at San Francisco's Exploratorium for help with this article. Check out their rock candy recipe with notes at exploratorium.edu.