Why is it that freshly brewed coffee tastes extremely different from coffee that’s been sitting around, even if it’s reheated? What, exactly, happens to the coffee to change it?
Also, does the amount of caffeine change over time?
The caffeol, the volatile oil mainly responsible for giving coffee its flavor, evaporates away. Once it’s gone, your cuppa Joe tastes like crap.
Simple oxidation prob’ly accounts for some of the differences; however I suspect that the evaporation of the lovely oils etc. that provide the heavenly aromas and tastes.
I do not think that the caffeine content is changed other than through the evaporation of water from the coffee.
Dude, how do you have so many posts?
They’re cumulative.
Hey, you probably haven’t even read my question, but I want to apologize for its (my) stupidity…I’ve read some of your posts and I’m really impressed with how much you know.
I did read it, actually, but no offense was taken–I’ve often asked myself the same question. But, thanks for the kind words. 
To be honest, the bulk of my posts were made early on when I was a new member here, and unemployed at the time (short version of a long story: quit a job I hated); back then, I had lots and lots of free time to post. Nowadays, not so much.
More important, Q.E.D, are you truly a “dude”?
Define “dude.”
Not fair!
Make ThomasHanks do it. 
If it hasn’t been too long, and the coffee’s still pretty warm, re-heat it to just barely drinking temperature and it won’t taste too bad.
Sitting around, though, is probably too long.
mangeorge