system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come a soft cookie becomes hard when it is stale while a hard cookie gets soft?
gpt
Because they adjust to the humidity of the surrounding air. The "humidity" of fresh soft cookies is above the humidity of air, so they get drier with time. However, the humidity of fresh dry cookies is lower than the humidity of the surrounding air, so they get moister with time.