Take a potato and slice it as thinly as possible (hard to do, but easier with a good knife). Then you spray a microwavable plate with non-stick spray, arrange the slices on it, not touching each other, and then microwave them until they start to turn brown around the edges (can take around 10 minutes, but watch them closely because they will burn). When they’re done, allow them to cool and become crispy. Then salt and eat.
I’ve never tried it with sweet potatoes, but I guess it would work. It’s kind of a tedious process, but it’s good when you’re craving junk food and you haven’t got anything healthy in the house.