system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do bath towels start smelling bad when all they do is fill with water and then dry?

I'm guessing maybe the longer drying time means mildew has a chance to build up? Or maybe when you dry off, your body isn't perfectly clean so the towel is picking up soap and dirt from your body which then builds up and smells?
gpt
You are surrounded, all the time, by millions upon millions of tiny microorganisms.  You are also coated in them, even after you shower. 

Getting your towel wet makes it a hospitable environment for this host of creatures. Many of them, when they become numerous enough, smell pretty funky.