Want proof that food can change your mood? Think Thanksgiving, the meal that leaves you as sleepy as a dose of Valium. Then there's the grande mocha latte; does anything else pick you up quite like a drink that combines two caffeine-saturated foods in one foamy cup? And when you're feeling low—your hard drive just detonated, your credit cards are maxed out, your college freshman just arrived home, mid-semester, with a moving van—you don't reach for broccoli, do you? No, you grab chocolate, because that's what makes you feel better.