氦气球为什么能自己飘起来？

A helium balloon floats up by itself. 氦气球，一撒手就自己往上飘。 Helium gas is much lighter than air. 氦气比空气，轻得多得多。 Light gas wants to go up. 轻的气体，总爱往天上跑。 It pulls the thin balloon skin along. 它拉着薄薄的气球皮，一路向上。 Air pushes the balloon up, like water pushes wood. 空气往上托它，就像水托着木头。 Never let it go near fire. 千万记住，别拿它靠近火哦。 One day the gas leaks out and it drops. 日子一久，气漏光了，它也就掉下来。