Rock candy, which is composed of sugar crystals, is one of the oldest forms of candy. It is exciting for children to make, because the progress of the crystal formation can be followed day by day, building anticipation for the finished candy. The production of the candy also provides a teaching experience, showcasing the process whereby a supersaturated solution--in this case, sugar and water--is unstable, and evaporation will cause the crystals to precipitate or form back into a crystalline structure with very little provocation. The process is usually slow, but it can be sped up with the right stimuli.
Heat 2 cups water over medium heat in a saucepan until it comes to a boil.
Stir 4 cups sugar into the boiling water with a wooden spoon until the sugar dissolves and the solution turns clear. Continue stirring until the solution again reaches a rolling boil.
Remove the saucepan from the burner, and pour the solution into a heavy tempered jar, such as a canning jar.
Tie the heavy nut to the end of the string, and tie the other end to the pencil.
Speed up the formation of the sugar crystals by dipping the string into the sugar solution, coating it with sugar and allowing it to dry out for 24 hours. When the string is suspended in the solution, it gives the sugar crystals in the solution something to attach to. Balance the pencil across the top of the jar, and drop the heavy nut into the solution, straightening the string.
Locate the jar in a warmer part of the house, such as near a heating vent, and also position a fan to blow over surface of the jar to continue to speed up the formation of the crystals. Warmth and moving air speed up evaporation of the water in the solution, causing the crystals to precipitate or form into crystals faster than normal. Make sugar crystals in three to five days instead of the usual seven days or more.
Food colouring can be added to the solution to create coloured candy. If the solution gets on your skin, immediately run cold water over the affected area to cool the syrup. Seek medical help if necessary.
Be extremely careful when boiling and pouring candy. The heavy syrup will adhere to skin and create very bad burns. Children are fascinated by the candy-making process and will want to watch closely, but do not allow them to come close when boiling or pouring the solution.