When you stand the bottles in hot water you make the air expand.The heated air takes up more room than the room temperature air and so when the heated air expands it forces the balloon to inflate.

• 所以当你再将瓶子放到热水中时，热水使得空气膨胀，加热的空气比室温下的空气佔据更多空间，因此当加热的空气膨胀时，气球就因此充气了。