The quest for a magic diet pill isn't new, of course. Scientists have been searching for a miracle drug that can carve out six-pack abs for decades. Some modern concoctions claim to speed up your metabolism. Others suppress appetite. One less-than-ideal option works as a diuretic. The new Gelesis100—which sounds more like a video game console than a drug—takes a different approach: The capsule is consumed before meals with a glass of water, and then temporarily swells up in the stomach, causing you to feel full and eat less.