Hydration 

The popular opinion about staying hydrated is to take at least 8 glasses of water in a day. 

According to experts, there's no evidence to back up this long standing statement. And the institute of medicine recommend that men have about 125 ounces of water per day and about 91 ounces for women. 

This recommended intake includes water from foods and beverages. Ideally, people get enough hydration unless they are exposed to heat, stress or they are physically active for a long time.