When you blow up a balloon, you push air inside. Then, as the balloon
gets bigger, it pushes the air around it to make room for itself. When
you try to blow up a balloon inside a bottle, one of the first things
that happens is that the balloon expands to touch the neck of the
bottle and it makes a seal, locking in the air which is inside of the
bottle but not inside the balloon. There's nowhere for this air to go,
so its pressure increases as you try to blow up the balloon and it
becomes rapidly very difficult to force more air into the balloon.
If you put a straw in between the balloon and the bottle, it opens
a pathway alongside the balloon in the neck for the air in the bottle
to escape. The air in the bottle that's not in the balloon now just is
at atmospheric pressure, so it is not that hard to blow up the balloon.
Of course, it may be possible to crush the straw between the
balloon and the bottle's neck, and then the first situation applies
(the seal may be leakier though).
Adam (and Tom)
(published on 10/22/2007)
