Good old-fashioned water is an American classic. What it lacks in excitement it more than makes up for in convenience. It's hydrating, versatile and it flows freely from the tap. Plus, it already makes up two-thirds of your body, so you know you can trust it. Not to mention it covers 70 percent of the Earth's surface so it's fairly easy to find, too. But how much should you be drinking during hard activity? Most doctors advise drinking two to three eight-ounce cups of water in the two-hour window before you exercise eight ounces every 15 minutes during exercise. But if you want to get your intake down to a science, try weighing yourself before and after your workout. If you weigh more after, youre drinking way too much water and should probably stop doing keg stands over your Camelbak. If youve dropped a couple pounds, take in two cups of water for every pound lost.