When making jacket potatoes or baked potatoes, where you bake them with their skin on, you want the skin to become crispy. The way to achieve this is by the use of crushed salt, which will draw out some water from the surface of the potato and therefore make the skin crunchy. The same method is used for crisping up other foods, like pork or other meat. To do the crispy potato, first prick it with a fork all over, then rub it in oil, and then cover it in salt. Then bake the potato until you can easily stick the fork into it.