system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly causes thirst and why do humans feel instantly better after drinking liquids when thirsty, even though not enough time has passed to rehydrate the body?
gpt
You lose **a lot** of moisture through your airway. When you get dehydrated it is the first thing your body tries to shut down to conserve water.  The mucus and tissue in your mouth and throat dry up, among all the other symptoms.

When you drink water it does immediately refresh the tissues and the mucus (like pouring water on a booger), making it easier to breathe and swallow, and relieving some of the pain associated with those dry tissues.  Also, we have evolved to associate those feelings with the urge to drink water, so just like eating when you're very hungry provides an instant reward, so does drinking water when you're thirsty.  If you have other effects from severe dehydration, those take a long time, but everyday thirst primarily manifests just in the mouth throat and lungs.