system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why when you drink water when you're really thirsty you feel better/hydrated instantly but in that moment hydration hasn't even started?
gpt
The brain rewards you for behaviors that keep you alive. When the brain notices that you need water, it triggers the "thirsty" signal. As soon as you drink something, the brain rewards you for what you did and gives you that "ahh, hydration" feeling. What matters to the brain is that you actually did the life-preserving action, no need to wait and see whether the drink actually hydrated you.