Empty a 1L pop bottle and make a small hole in the side.
Place a balloon inside and wrap its open end around the top of the bottle.
Apparently the balloon won't inflate if you cover the whole in the bottle with your finger because the air in the bottle is exerting a pressure on the balloon. I would have thought that you could still inflate the balloon and the air in the bottle would be compressed?
Also once you have inflated the balloon (by uncovering the hole) the balloon will say inflated when the bottle is removed from your mouth (if you keep your finger over the hole). Why does it stay inflated?
Why will it then deflate when you move your finger off the whole?