Water is a kind of chemical substance.

Every year there are reports of people dying as the result of extremely hot weather．Many of the victims are old persons，whose hearts or breathing systems decline．But many die from lack of water．
   Water is necessary for life and good health．We often forget this fact when we think about the other building blocks of life such as vitamins，minerals and proteins．We can live for many days without eating，but two or three days without water usually leads to death．
   The human body may look solid．but most of it is water．New born babies are as much as 85% water．Women are about 65% water and men about 75%．Women usually have less water than men because women，in general，have more fat cells．and fat cells hold less water than other kinds of cells．
   Water does many different things to keep us healthy．It carries hormones，antibodies and foods through the body，and carries away waste materials．That is why different parts of the body contain different amounts of water．For example．blood is 83% water．muscles are 75% water，the brain is 74%．and bones are 25%．
   Water is also necessary for cooling the body under hot weather and when we are working hard or exercising，water carries body heat to the surface of the skin，where the heat is lost through perspiration．Researchers say cool liquids cool us faster than warm liquids，because cold liquids take up more heat inside the body and carry it away faster：They say，however，that cold sweet drinks do not work well because the sugar slows the liquid from getting into the blood-stream．
   Researchers also note that fat cells block body heat from escaping quickly．Fat cells under the skin act like warm clothing to keep body heat inside．This is why overweight people have a more easy time staying cool than thin people．
   The body loses water every day through perspiration and urine．If we lose too much．we will become sick．A 10% drop in body water can cause the blood system to fail．A 15%～20% drop usually leads to death．To replace what is lost，health experts say growing persons should drink about 2 liters of liquids each day，and more in hot weather．They say we can also get some of the water we need in the foods we eat．Most fruits and vegetables are more than 80% water．Meats are 50%～60% water．And even bread is about 33% water．Water may be one of the most simple of all chemical substances，but it is the most important substance that we put into our bodies．