WATER

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It helps to Improve Mental Performance 
Approximately 80 percent of your brain tissue is formed of water; therefore, the hydration of your brain is essential. Dehydration impairs concentration and reduces the short-term memory. Ample amount of water in the brain helps in reducing the thickness of the fluids carrying enzymes and proteins and in delivering these nutrients to their destination. If the body is dehydrated, it slows the process of these fluids which directly affects the mental performance. 

Prevention of Critical Ailments 
If you drink enough water, you may not be affected by a majority of serious illnesses including cancer, arthritis and heart diseases. As there is plenty of water in your system, there will be less friction in your joints and consequently less chances of developing arthritis. It also helps in flushing the toxins out of your body. Over time there will be a reduction in the amount of toxins in your body that come in contact with your bladder and colon. This will decrease the chances of formation of cancer. In addition, excessive salt is washed out. 

Helps in Losing Weight 
Drinking water speeds up the metabolism in your body. Every process that occurs inside your body requires water and eventually slows down when it is thirsty. Pouring more water will lead to a faster metabolism which in turn will burn calories at a faster rate. According to a study, if you take two cups of water, it takes around 10-40 minutes for your metabolism to kick-start. Once it does kick-start, you will be able to burn calories at a higher rate. 

Cavities are Washed Away 
Drinking plenty of water will keep you irrigated which will help produce many important fluids including saliva which washes off cavities. Saliva contains minerals that help in tooth repair and it also neutralizes the acid which causes tooth decay. 

BUT DONT OVER DRINK

Excessive Water May Lead to Over-Hydration 
It's important to replenish the body with water that you lose everyday, but don't go overboard and drink too much of it because that may cause over-hydration. There is an over-dose of water in your body when you drink more water than that is expelled. When this happens, sodium levels in your body become unbalanced which in turn may lead to seizures, digestive problems, or may result in a coma. A little under a gallon of water for an adult is more than enough for a day. 

(Content from http://sg.news.yahoo.com/photos/5-amazing-facts-water-slideshow/)