Weight gain happens when you eat more food than your body needs to stay healthy and strong. Imagine your body as a little car that needs fuel to run. Food is like the fuel for your body. You put the fuel (food) in the car (your body), and it gives you energy to play, learn, and do all the things you like.
But sometimes, you eat too much fuel. Maybe you have some delicious cake, candy, or chips that you really like, and you forget to stop eating. If you eat more fuel than your body can use, it stores the extra fuel as fat. Fat is like a little storage unit where the body puts the extra fuel.
Over time, if you keep eating more fuel than your body needs, the little storage unit (fat) can get very full. That's what leads to weight gain. Imagine you have a backpack that you carry with you everywhere you go. Every time you eat too much fuel, you add a little more weight to your backpack. Sometimes, it's ok to have a little extra weight in your backpack, but if it gets too heavy, it can be hard to carry around.
That's why it's important to eat healthy foods and exercise regularly. Eating healthy food gives your body the right amount of fuel it needs to stay healthy and strong, without storing too much extra fuel as fat. And when you exercise, you use up some of the extra fuel your body has stored, which can help you maintain a healthy weight.