Do you know any people who never gain weight, no matter how much they eat? Some people are lucky like that. They can eat pizza, fries, and just about anything else without seeming to gain an inch. Exercise aside, a person's metabolic rate determines how hard or easy it is for them to gain weight. The metabolic rate is different for each person. That's why some people never seem to gain any weight, even if they eat a lot and get very little exercise...