system
Given the following post, respond as if you're commenting on Reddit:
human
How does water go stale?

Ever drink from a glass of water that had been sitting out for days? It tastes weird, or stale. Why is that?
gpt
Water doesn't "stale" so much as it stagnates. If you leave a bottle of water in an environment that is favorable for the growth of microbes, you're going to end up with a nice soup of bacteria, protists, and other nasty critters that you probably wouldn't want to drink. 
 
To avoid this, don't leave out water at room temperature after you've already taken a drink of it (and smothered it with your saliva-covered lips). Put it in a refrigerator, or something. Water that has never been contaminated as such can be left out perpetually until it is opened and drank.