Step 4: The Science
Sugar naturally has a crystalline structure, which would be noticed under a microscope with the individual grains.
A solvent (in this case, water) can only dissolve a certain amount of a solute (in this case, sugar) before it becomes fully saturate.
By heating the water, it is able to dissolve more and more sugar. This becomes known as a supersaturated solution.
We allow the water to absorb the most sugar it can by heating it to it's boiling point of 212°F (100°C), but try to avoid caramelizing and burning the sugar.
As the water cools, it can no longer hold the same amount of sugar we added. This sugar is "seeded" on the skewer or string, but will not grow on the clean glass surface or perhaps a nylon string or fishing line. The sugar "regrows" into a crystalline structure.