A balloon expands as you blow into it because of the increase in pressure inside the balloon caused by the air you are blowing into it. When you blow air into a balloon, the air molecules inside the balloon are pushed closer together, increasing the pressure inside the balloon. As the pressure inside the balloon increases, the balloon stretches and expands to accommodate the extra air.
The balloon is made of a flexible material, such as rubber or latex, which allows it to stretch and expand as the pressure inside it increases. The balloon's material has elastic properties, which means that it returns to its original shape when the pressure is released.
The process of blowing air into a balloon can be compared to filling a tire with air. As you inflate a tire with a pump, the air molecules inside the tire are pushed closer together, increasing the pressure inside the tire. This causes the tire to expand and become larger, until it reaches the desired pressure.
In summary, a balloon expands as you blow into it because of the increase in pressure inside the balloon caused by the air you are blowing into it. The balloon's material has elastic properties, which allows it to stretch and expand as the pressure inside it increases.