However, many people can not stop themselves from eating, and they become fat or obese. You can become fat by eating the wrong, unhealthy types of food and by eating too much. When a plant collects energy from the sun through its leaves, it is called photosynthesis. If a plant gets too much food, it can just store it until later. They use this stored food during the winter. When you are proven to be fat or obese through your body mass index (BMI), it means you are heavier than the average for your height. The taller you are, the heavier you should be, in theory. If someone is too heavy to be called fat, then they are called obese. When someone eats too much of one thing, they become fat. Someone can eat junk food, but they can also balance themselves by also eating fruits and vegetables. 