system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how come a cup of coffee doesn't seem to quench your thirst, even though it's mostly water?
gpt
It's because of tannins (and pseudo-tannins), compounds that often have the same effect in red wine and tea. They bind to proteins in your saliva, and make those proteins aggregate in a way that your saliva becomes less lubricating for your mouth. This is called astringency.

You can make coffee less astringent by brewing it less hot/for less time.

Eating cheese or other fatty foods apparently also reduces astringency.