On the show, you see Walt and Jesse cooking, transporting, and selling meth on a regular basis. Obviously, the production designers can't use actual crystal meth on the show, so instead they use rock candy!
One of the most interesting things about the set is that the equipment that Walt and Jesse use is legitimate lab equipment that could plausibly be used to cook meth. Instead of using it for such a nefarious purpose, however, the producers really do use it to make the rock candy that stands in for meth on the show.
