Question

As we blow air into a balloon it becomes bigger and bigger why

Answer 1

With more air molecules in the balloon, the pressure inside the balloon increases. The balloon inflates to increase its surface area so that the force acting by the air molecules inside outward = the force acting by the air molecules outside inward

Answer 2

Air molecules collide into the balloon with the same energy inside and outside theballoon. When the bottle is heated, the air molecules inside start moving faster. These molecules now collide into the balloon with more energy resulting in increased pressure. The increased pressure causes the balloon to expand.