Homemade rock candy is easy, fun, and illustrates the scientific concept of crystallization.
It also teaches patience, because you have to wait a long time before you can actually eat the stuff.
First get a jar, a pencil, and some twine. Tie the string to the pencil and prop the pencil over the mouth of the jar so the string hangs down into the jar, reaching the bottom without bunching up.
You’ll want to attach a nontoxic weight to the bottom of the string (say, a stainless-steel paper clip) so that it hangs straight down. Heck, since you’re going to the trouble, you might want to prepare several jars and double or triple the recipe below.
In a saucepan, bring a cup of water to a boil. Slowly stir in two cups of sugar, adding half a cup (or less) at a time and waiting for it to dissolve completely before adding the next batch.
When all of the sugar has been dissolved into a thick, clear syrup, pour the mixture into your empty jar. Make sure the string is submerged into the hot liquid. Put the jars in a place where they won’t be disturbed or tipped over.
Because the water mixture was supersaturated with sugar, the two couldn’t stay mixed except when very hot. As the mixture cools, the sugar will begin to glom onto the string, creating cool crystal patterns. As the water evaporates further over the coming weeks, the crystals will grow bigger and bigger.
If you want to play a sadistic practical joke on someone, you can surreptitiously make a special batch of similar-looking “rock candy” by using salt instead of sugar.