Running out of steam is such a great metaphor for physical exhaustion, because of how often dehydration is the reason I'm exhausted. (It's so easy to forget to drink water. Do you know how easy it is to forget to drink water? Because it's SO EASY.) Because steam engines run on water. So not having water would cause them to run out of steam.
I've had this topic in my phone for like a month, because I figured it'd be a good post for a day when I'm exhausted and can't think of something else to write about.
I think I may have underestimated the value of the metaphor.