In a medium saucepan, heat 2 cups of the sugar and the water. Stir until the sugar is completely dissolved. Gradually add a few drops of the food coloring of your choice and the additional sugar, stirring continuously until all the sugar is dissolved. Pour the solution into a clean glass jar and tie the pieces of string to the pencil and suspend them across the mouth of the jar so that the ends hang into the sugars water. Crystals suitable eat will form in an hour and continue for several days to a week. Pieces can be broken off and eaten after the first hour.