What You'll Need:
Pencil or other thin piece of wood (long enough to fit across the mouth of the jar)
Cotton string with a rough surface
3 cups sugar
1 cup water
Spoon (plastic or wood is best)
Food coloring (if you want)
Cut a piece of string about 8 inches long. Tie one end around the pencil. Place the pencil across the mouth of the jar, with the string hanging down inside. The string should reach almost to the bottom of the jar, but shouldn't touch the bottom or sides. If it's too long, cut the string or wrap it around the pencil to shorten it. Put the pencil and string aside.
Put the water in the pan and boil it. When it is boiling, start to add sugar, about one teaspoonful at a time. Stir until the sugar dissolves completely, then put another teaspoon in and stir that until it dissolves. Keep adding sugar until it won't dissolve any more (you may use less than 3 cups). You need to get as much sugar as possible into the solution in order to get big crystals quickly. There shouldn't be any undissolved sugar on the bottom of the pan. If you want colored rock candy, add a few drops of food coloring at this time.
Carefully pour the sugar solution into the jar (remember that it's very hot!). Make sure that there are no sugar granules in this solution. Put the pencil over the mouth of the jar as before, and let the string hang down into the solution. Place the jar somewhere that it won't be disturbed. Cover it with a paper towel to keep dust out.
Now you wait! The shiny sugar crystals will grow slowly, and you should see them starting in a day or two. It will look something like the picture on the right. In about a week, you'll notice that the crystals aren't getting any bigger. That's when you can take them out of the jar to dry. Before you enjoy eating your rock candy, take a close look at it. What shapes do the sugar crystals have? Is this the same shape that the original sugar granules had?