Imagine that you are running in a park on a very hot day． Will you feel tired？ The answer is yes．But besides （1） 　　 feeling of being tired， I believe most people will feel thirsty soon． You will feel much（2） 　　（good） right away after you drink some water． Your body needs water， so it tells you to drink． It's simple， right？

Actually， it's not（3） 　　（ real） simple． There is a special part in our brains．When our blood is too salty， it says，"Drink!" （4） 　　we drink， it is quiet． Water does not go into our blood right away． However， our brains still know that we（5） 　　（ drink） before． How？ When you swallow（吞） fast， your brain knows and thinks you are drinking water．

If you drink something else， your brain will still be quiet． It will still think you are drinking water．Maybe you drink cola or even oil． You will stop（6） 　　 （feel） thirsty． But in fact， drinking those things is not good （7） 　　you when you feel thirsty． You should just drink water because your body needs it most．

How much water should we drink each day？ Here are doctors' （8）_____ 　 （ opinion）．Some doctors advise（9） 　　 （we） to drink eight cups of water every day，while other doctors say that's too much． On a hot day， you lose a lot of water． So， maybe you should drink more． Every person's body has（10） 　　（ difference） needs．