How and Why
Moisture is the culprit when discussing stale potato chips. The chips themselves are very dry due to the fact that they have been fried and rid of almost all moisture. When a bag of chips is left open, the air in the room circulates into the bag. When the air comes into contact with the dry chip, the chip will soak up some of the moisture in the air. The longer a bag is left open, the longer the chips are exposed to the moist air. Stale chips are basically chips with too much moisture. They lose their crispness.
But bags of chips come with air already sealed inside, so how are they not already stale when they are opened? The reason is that they are sealed with air in a factory that maintains a low humidity level.
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If you do have stale chips, you can revive them to their former glory. All you need is a microwave or oven. In a microwave, set for about 20 seconds and then let cool. An oven is only practical if you need to revive quite a few chips. In this case, spread them out on a pan and bake at about 350 degrees for about 5 to 10 minutes. Check frequently to make sure they don't overcook. In the case of the microwave and the oven, let the chips cool before you eat them.