Describe how the gas particles inside the balloon exert pressure on the balloon

as you pump more gas into the balloon, the particles become more packed, in an attempt to gain more room, the particles push on the walls of the balloon, therefore, making it expand.

As you put gas into the balloon, more air particles go inside. As that happens, the particles all push on the sides of the balloon, making it expand larger. If you wanted to deflate the balloon, you would probably poke a hole in it. As you did that, the air particles escaped from the balloon and into the air.