Rock candy looks like sparkling, sugary diamonds. This is an excellent candy-making project to do with children, and a good lesson on crystallization.

Punch 7 or 8 holes ½ inch up 2 opposite sides of an 8-inch square foil pan, and lace kitchen twine from one side to the other. Place the laced pan in a larger pan at least 1 inch deep to catch leaking syrup. Combine in a medium heavy saucepan:

Stir over medium heat to dissolve the sugar. Wash down the sides of the pan with a pastry brush dipped in warm water. Increase the heat to medium-high and cook, without stirring, until the mixture reaches 250°F, the hard-ball stage. Remove from the heat and, if desired, quickly stir in with a wooden spoon:

Carefully pour the syrup into the laced pan. Do not scrape the bottom of the saucepan. The syrup should come ½ to ¾ inch over the strings. Cover the pan with a piece of plastic wrap, so that you can see what’s happening without jostling the pan. Place in a warm, dry, draft-free place, such as an oven. Watch and wait. Within 36 to 48 hours, you should see crystallization begin. When all the syrup has crystallized, lift out the laced pan. This can take days. Cut the strings and dislodge the rock candy from the pan. Put on a baking sheet in a 200°F oven to dry for 2 hours. Store in an airtight container at room temperature.