热气球为什么能飞上天？

Hot air rises up, up, and up. 热空气呀，一个劲往上升。 It is lighter than the cool air around it. 它比周围的冷空气要轻。 A balloon traps the hot air in a big bag. 热气球把热空气装进大口袋。 The light air pushes the bag up. 轻轻的热空气，把口袋往上托。 A burner keeps the air inside hot. 燃烧器一直把里面的空气烧热。 When the air cools, the balloon goes down. 空气一凉，气球就慢慢落下来。 That is how people float high in the sky. 人们就这样，飘在了天空里。