HOW COME?

Air trapped behind the balloon can only be "squashed" or compressed so far and then it "pushes" back preventing the ballon getting any bigger. The pushing of air is called air pressure. When air pushes against something it is called pressure.

Air pressure is all around us as we live under a "sea of air" - a bit like a fish surrounded by a sea of water. The air presses on us from all side, but we are so used to it we don't feel it. Every part of our body is pushing back (each cell is like a balloon) so we don't get squashed flat.

If we travel in an plane, the higher up in the sky (atmosphere) we go, the less air pressure there is. If we go into space where there is no sea of air (no atmosphere) our bodies (our cells) still push outward but have nothing to stop them - we explode!

This is why astronauts have to wear a space suit, not only for air to breath, but to contain air at the correct pressure to push back against their bodies.