Water

Your body is at least 70% water.  That fact alone should indicate to you how critical water is to the health and proper functioning of your body.  During exercise, as your body temperature rises, your body perspires, or loses water.  This will lead to dehydration if the water is not replaced.  Dehydration leads to a breakdown in the body's ability to function correctly.  For this reason it is critical that one re-hydrates throughout an exercise session.  It is a good idea to drink water or sports drink every 5 to 10 minutes while exercising.