人类总想像鸟儿一样地飞行。鸟儿之所以能轻易地飞起来,是因为它们的体重轻,而人类无法飞起来,则是因为他们的身体太重。人类第一次升到空中是借助气球来实现的,这些气球是充满气体的大袋子。氢气对气球来说是一种有用的气体,它比空气轻许多,氮也是一种比空气轻的气体,但因为它太昂贵,因此气球通常都是用氢气来充填。 Human beings always fly like birds. Birds fly easily because they are light and humans can not fly because they are too heavy. The first human ascension into the air was done with the help of balloons, large balloons filled with gas. Hydrogen is a useful gas for balloons, which is a lot lighter than air. Nitrogen is also a lighter gas than air, but because it is too expensive, the balloons are usually filled with hydrogen.