Weight gain occurs when the body's energy intake (usually measured in calories) is greater than its expenditure of energy. The body has to do something with the excess energy, so it saves it for a rainy day as fat.

Thanks to changing food marking and manufacturing practices, there are more and more calories to be had. Portion sizes have dramatically increased, at fast-food chains and everywhere else. Foods are often made with high fat content (think greasy hamburgers with a side of fries), and even those without much fat are often overendowed with carbohydrates, in large part because of the wide availability of high-fructose corn syrup, a cheap sweetener that lacks any nutritional benefit except calories.

On the energy-expenditure side, we are also headed in the wrong direction. Many schools have reduced physical-education requirements. Because of cars and elevators, we walk and climb less than we once did. Increased automation means that fewer jobs are physically demanding. And the easy availability of passive entertainment (television) also makes it easier to save ingested calories as fat instead of burning them up.