Getting enough water every day is important to your wellbeing and necessary for your organ functions. In the Journal of Biological Chemistry, the human adult body is composed of 60% water. Your brain, kidneys, heart and lungs need around 80% of water to function, and your skin needs to be composed of 75% water to maintain its elasticity. As your breathing, digestion, and waste removal depend on your constant rehydration, it is no wonder that you feel lethargic when you're thirsty. Many experts also go as far as saying that you need water more than food if you're lost in the wilderness.