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.
We can see what will happen to a helium
filled balloon if it rises too high in the air. The higher up in the
sea of air (atmosphere) the balloon goes, the less air pressure there
is. The gas in the baloon still pushes outward even though there is
very little air pressure push back. The balloon swells and expands due
to the greater air pressure inside compared to the outside...until it
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. Find out about sea divers and "the