The best thing to drink to keep you hydrated is water. It replaces fluid lost through breathing, sweating, urination, and everyday activity without adding sugar, caffeine, or unnecessary calories.
Water is vital because it helps regulate body temperature, transport nutrients, cushion joints, and remove waste. The body also gets water from foods such as fruit, vegetables, soups, and dairy products, but plain drinking water is the simplest and most generally suitable source for hydration.
People sometimes assume that coffee, tea, or sports drinks cannot hydrate the body, but moderate amounts of caffeinated drinks still contribute fluid. Sports drinks may be useful during prolonged, intense exercise because they contain electrolytes and carbohydrates, but they are usually unnecessary for ordinary daily activity. During illness, extreme heat, or heavy exertion, fluid needs can rise, and medical advice may be appropriate.