The process of gaining or losing weight can be explained / by comparing your body to your car. / Both run on fuel，food for your body and gasoline for your car. / Both of the fuels must first be converted into heat, then energy, / some of which is used to do work, / and some emitted as waste, and just as your car uses more energy / when the engine is racing than when it is idling, / so does your body use more energy when you are working harder than when you are resting. /

 

However, there is one significant difference between them. / Your car cannot store fuel / by turning it into something else； all gasoline not consumed remains as gasoline. / But your body stores excess energy as fat. / When the gas tank is completely empty, / the car won’t run； / but your body can burn fat to provide more energy. / Therefore, if you want to gain weight，/ you must do either of two things： / eat more calories, / or use less through inactivity. / If you want to lose weight, / you do the reverse ： decrease your intake of calories or increase the amount of energy you spend- / There is no other way, gaining or losing weight is always a relation / between intake and output of potential energy. /