To stay healthy, it's important for the body to replace fluids lost through breathing, sweating, and urination, as water makes up about two-thirds of a person's healthy weight and is needed for most chemical reactions in cells. Drinking water is the healthiest choice to quench thirst as it contains no calories or sugars and helps blood carry nutrients and remove waste, though the right amount depends on factors like size, temperature, and activity level. We obtain some water from food but most from drinks including water, milk, juices, and tea or coffee, while alcohol does not contribute.