You've probably seen them on "60 Minutes": 800-pound giants so trapped in fat that they can barely walk. While people that big are a rarity, many Americans do suffer from obesity.

Although hormonal imbalance and genetics play a role, there is no one reason why people become obese. Some evidence suggests that early eating habits may lead to children becoming dangerously overweight. More generally, a national life and culture that offers an abundance of high-calorie foods and promotes a sedentary lifestyle can lead to widespread obesity.

How and why we eat doesn't seem much of a mystery: we eat when we're hungry and stop when we're full. But as anyone who's ever eaten an entire bag of potato chips in one sitting knows, it's not that simple. When food enters the stomach, it triggers receptors that alert the brain to induce a feeling of fullness.

Fat, especially, triggers this sensation. But fat also triggers receptors in the mouth and brain that urge us to gulp down food before we feel too full. It's a battle between forces trying to make you feel hungry and full, including a host of social conventions and personal appetites affecting your diet. When the balance is upset and the various forces of appetite consistently win out, obesity may be the result.

Because obesity causes a range of serious medical problems including diabetes and shortened life expectancy, weight loss becomes crucial. Diets abound, but no one has proven entirely effective. There's even surgery that shrinks the stomach and reduces the amount of food digested by the body. Whatever it takes, the goal is to eat better and eat less. That's the only way out of obesity.