Helium is lighter than air and just as the heaviest things will tend to fall to the bottom, the lightest things will rise to the top. Helium weighs 0.1785 grams per liter. Nitrogen, which makes up 80% ...

放飞气球在很多人看来都是一件很浪漫的事情，但是你知道放飞气球的危害吗？氦气球飞到空中会杀死鸟儿，落到地上会呛死牲畜，掉到海里会害死海龟，而且还会严重污染自然环境。所以，放飞气球仅仅是看上去很美，而已。 Release a helium balloon on a bright, sunny ...