Diffusion is the answer.
In general, the concentration of gases and liquids always wants to even out in the area they are in over time.
Fresh bread is a very moist food, and contains more water than the air, so as it is exposed it loses water vapour into the air, causing it to dry out and become hard.
Potato chips are crispy and are a very dry food, and so have a lower water content than the air, therefore they GAIN water from the air, causing them to soften.
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