Fill a saucepan with 2 cups of water. Bring it to a boil on medium-high heat. Pour 4 cups of sugar in the water and stir constantly until it dissolves. Remove the saucepan from the heat. Carefully pour the mixture into a glass jar. Tie a washer, ring, clothespin or weight to a piece of string measuring two-thirds of the jar's height, tying the other end to the center of a pencil or wooden skewer. Dip the string into the sugar, then lay it out straight and allow it to dry for two to three days.
Hang the pencil or skewer atop the jar, letting the string sit in the sugar mixture for five to seven days to allow for crystal growth. Record how different the candy looks each day. Include notes on crystal shape and size. Carefully remove your rock candy when the crystals are fully formed. Study and describe your results. Add food coloring to your solution and note any differences during the candy formation process.
If possible, take photos of your rock candy each day so you can study it in detail, even after you've made initial observations. Keep the photos numbered and in chronological order with their notes attached so you don't lose them. If you make one piece of clear rock candy and another colored piece, note their similarities and differences throughout the process. You can also try corn syrup, molasses and other types of sugar to see if and how crystals form with each type of sugar.
When you mix one part water with two parts sugar, you create a saturated sugar solution, so no more sugar can dissolve at a certain temperature. This solution is unstable because it contains more solute, or sugar, than liquid, so the sugar actually leaves, or precipitates from, the solution. Water also evaporates over time, adding to the process and spurring more crystal formation. Finished rock candy has about a quadrillion molecules on a single string.