We're always told that it doesn't matter what we eat, as long as we control our calorie intake: Eat less, move more, and we'll be fine. If that doesn't work for us, then we're just too greedy and too lazy. But what if it's not that simple? Our body chemistry has a whole arsenal of 'chemical weapons' that make us go in search of just the foods we're trying to avoid. Willpower doesn't even get a look in.