If you’ve ever paid attention to health topics, you’ll probably be impressed by the widespread saying that to stay healthy, drink (at least) 8 glasses of water a day. On the Internet, there are even some “work and rest schedules” for drinking water, which clearly arrange the time and efficacy of drinking each glass of water, such as “drinking the first cup at 6:30 in the morning to clean up the intestines and stomach; drinking the second cup at 8:30 to improve vitality…” are we not even free to drink the simplest water? To this end, global science had an exchange with Zhang Na, a public health expert at Peking University, hoping to help people interpret such health suggestions from the perspective of insiders.