Video narration
Watch this balloon. It looks empty, but as I blow into it, it grows bigger and bigger. What is filling it? Not nothing. Invisible air has entered the balloon and taken up space. The balloon changes shape because air fills the space inside it. Now press the balloon gently. The air is squeezed into less space, but it has not disappeared. It is still inside. Feel that push against your fingers? The stretched balloon pushes back because the air inside pushes outward on the balloon walls and your hand. That push is called pressure. So one balloon gives us three clues: air takes up space, air can be squeezed, and air pushes. Even when you cannot see it, air is doing something. That is why a full balloon feels firm in your hands. Remember the balloon: invisible air fills space, gets squeezed, and pushes back.