system
Given the following post, respond as if you're commenting on Reddit:
human
When my guitar is out of tune, the strings are always in a too high pitch. Intuitively, it feels like it should be the other way since they would loosen. What's going on?
gpt
The coefficient of thermal expansion of the (steel or bronze) strings is higher than that of the wooden neck of the guitar (along the grain).  After you've been playing the guitar for a few minutes, both strings and wood will have warmed up due to you touching them.

If the thermal expansion of each material were equal (along the string axis), then the guitar would stay in tune quite well.  Instead, as it warms, the strings expand more than the wooden neck, causing some tension to be lost.  This lowers the pitch of the strings.

If you tune your guitar perfectly in this warm state, when it cools, the strings will be tuned too high.

TL;DR:  Don't check your guitar's tuning until you've been playing it for 5 minutes.