Water is the first ingredient of life. In the comfortable environment in which we live, with an ample supply of water, we forget that our ancestors lived in an environment where water was scarce, and the weather was hot. We therefore developed a very powerful cooling system in which water plays a major role. The importance of this system is best illustrated when we are exposed to exercise and heat, separately and even more when both are combined. In these situations, the primary way to get rid of the heat generated or received from the environment is through the secretion and evaporation of sweat, which is mainly water. Thanks to this cooling system, we can sustain prolonged exposures to heat and we can work in the heat. However, if not properly replaced,fluid loss under the form of sweat results in dehydration. This reduces the ability to regulate body temperature as well as the ability to perform exercise. Under extreme circumstances, which fortunately are not often encountered, dehydration and the increase in body temperature can result in heat stroke, which could be fatal.