People tend to gain weight when their metabolism slows down. Metabolism is the ‘chemicals’ present in our bodies that break down the food that we eat and convert that food into energy. People tend to gain weight when their body does not convert the food into energy as fast as we eat it. Exercise is a way to prevent out bodies from becoming fat as we are constantly using energy from the food we eat. If we eat and do not work off that food and energy, we tend to gain weight a little bit at a time.