Go ahead, give it a try. Place a balloon into a bottle (a 2 liter soda bottle works well), leaving a bit sticking out to blow into. Give it your biggest breaths. What happens?
You'll get about this far:
Why can't you get it any more inflated? There's still room in the bottle and the balloon can certainly stretch further than that.
What's wrong?
It's that pesky air pressure again.
The bottle is filled with air. When you start to inflate the balloon, it seals off the opening of the bottle. The air molecules that were in the bottle are stuck in the bottle - they can't get out. And, you're trying to push more air molecules into that space as you inflate the balloon.
Unless you've got lungs unlike any person I've ever known, you don't stand a chance pushing against those air molecules in the bottle.
But, what if you gave those molecules an escape?
Try again, but this time, stick a straw in the bottle with the balloon - the end of the straw needs to be sticking out of the bottle.
Now blow (on the balloon, not the straw!)...
That's more like it!
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