I recall having varying success growing crystals as a child, and it's a project I've been meaning to do with my kids for quite some time now.
I believe you can grow crystals from pretty much any salt (Table, Epson and Borax are the most commonly available), as well as sugar. The science behind it, put very simply, is that the water can only hold a certain amount of your crystalline substance in solution, in other words, you can only dissolve so much in it until it becomes saturated. By heating the water it has the capacity to hold more, as essentially it's molecules have moved further apart. As your solution cools, the molecules move closer together again, causing your solid to crystallise out of the solution once more. The crystals are attracted to solid surfaces to reform, so you can encourage them to grow where you want them to by providing a foreign object within the solution. For our crystals we chose to use Borax, as the crystals are reputed to be fairly sturdy once they have formed, and we plan to use ours as decorations.
We used a pipe cleaner, an extra piece of wire, borax (which we sourced through ebay), a glass jar and a pencil to suspend our creation from.
We started by cutting our pipe cleaner into 3 equal lengths, and twisted it in the middle so it would hold its shape. You could use any shape or object, although I think having a rough surface for the crystal to attach to works best.
We bent a little loop on one of our arms, and twisted our extra piece of wire onto it so it held firm (remembering once you suspend your object in the solution it may try to float up if not held securely)
Using our jar as a guide we then worked out how long to leave the suspension wire so it would hold our start in the centre of the jar (you don't want it touching the sides or bottom)
We heated up 1 litre of water until it was very hot but not boiling and stirred in the borax a tablespoon at a time until no more would dissolve (in our case it was 10 spoons). We then simply poured into our jar and popped in our star. This photo shows our crystals after about 2 hours.
The star on the right was left overnight, whereas the one of the left was only in the solution for 3 or 4 hours. The crystals are fairly robust, over time they can become less sparkly, and it is suggested that you paint them with a clear nail polish to prevent this happening. I believe that by adding food colouring to your solution you can grow crystals of different colours, although we haven't tried this yet. The kids absolutely loved growing crystals, and can't wait to take them in for show and tell at school!