Water constitutes 50-60% of the total body weight in humans and plays many crucial roles in their physiology. First, it provides a suitable medium for most biochemical reactions in the body due to its excellent solvent properties. It also directly participates in many important chemical reactions, such as the breakdown of complex food molecules. Second, water is the main component of body fluids, such as saliva, blood, and tears. While saliva aids in swallowing and digestion of food, blood facilitates the transport of nutrients and elimination of toxic wastes from the body. Tears help to flush out irritants from the eyes and keep them lubricated. Similarly, water also serves as a lubricant in joints, protecting them from friction. Third, water has a high heat capacity that helps to regulate body temperature. It absorbs and redistributes the heat throughout the body via blood. When the body temperature rises, the heat is released from the body via sweat, which is mainly composed of water. This helps to bring down the temperature. Finally, water also acts as a shock absorber. For example, as part of the amniotic fluid, it helps to protect the developing fetus from physical trauma.