You will need: a jar, some boiling water, string, a spoon, salt, patience!
Stir salt into boiling hot water until no more salt will dissolve (crystals start to appear at the bottom of the container). Be sure the water is as close to boiling as possible. Carefully pour the solution into your jar. (putting a spoon into the jar before adding the water should prevent the jar breaking. Suspend your string into the jar from the spoon laid across the top of the jar. Leave your jar somewhere it will not be disturbed and wait for your crystal to grow!
Once your crystals have grown, you can look for some interesting things in them: