Combine high heat and high humidity with exercise or play, and you are in danger of dehydration and potential heat illness. That is why, no matter what your age or what shape you are in, it is important to drink plenty of fluids, especially water. If it came with an operating manual, the body would tell you that you need plenty of water. As you go about your day's activities, whether it is making breakfast, riding your bike, making a volleyball point or mowing the lawn, your body needs oxygen and nutrients. The blood stream, which is made up mostly of water, is the fuel line that delivers them.
Any activity causes the body to lose water.
As this happens, the body's cooling system work less efficiently. Imagine your body losing its coolant, a little every hour, like a car. Your car would overheat in no time. So does your body. The body feels the effect of dehydration in as little as a half an hour.