system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: When a guitar sits unplayed for a length of time, why do the strings tend to go out of tune to a higher frequency instead of "loosening up" to a lower one?
gpt
Most likely humidity. If the weather is more humid than the last time the guitar was tuned, the neck will have expanded/lengthened over that time so the strings will be tighter and sound sharp.  The opposite would occur if it’s less humid. If you tune regularly, you’ll notice you’re usually tuning down in the hot, humid summer and tuning up in the cold, dry winter.