Explanation:

Air is a gas; therefore its molecules fill the entire space of its container. When the air is heated, the temperature of the gas increases. The kinetic energy and the velocity of molecules increase with the temperature. As the molecules start moving with increased energy, they collide more quickly and more frequently with the walls of the container. If the container walls are rigid, then the pressure of the gas increases with no change in volume. But if the walls of the container are flexible (like a balloon), the increased energy of the collisions between the wall and the molecules causes the container to expand.