Drinking absolutely pure water can kill you. All the water we normally drink from taps, from bottles, from the garden hose contains impurities which affect a property of water called tonicity. This is the ability of a solution to draw or repel water from ajacent solutions when separated by a membrane. Solutions that are hypertonic gain water, while hypotonic solutions lose water. Drinking water, which we usually think of as pure, really contains a whole mess of salts and sugars which make it slightly hypertonic. They also give water its taste.
The cytoplasm in the cells of your body also have a tonicity. So, when they are exposed to hypertonic solutions, the cells themselves lose water and shrivel up. When they are in hypotonic solutions, they gain water, swelling up and bursting like a balloon. Absolutely pure water -with no impurities at all is the most hypotonic solution. So, if you drink enough of it, your cells will start absorbing water. This disturbs the balance of electrolytes (electrically conductive ions) in your body. This is most seriously a problem in the brain, where water poisoning can lead to brain damage, coma and even death.
My 8th grade science teacher tried that once, said it was the worst tasting water he ever had.
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