A cloud is a massive droplets in the sky. 云是天空中大量的小水滴。

　　When the sun heats up the Earth, water on the Earth evaporates and becomes a cloud. 地球受到太阳的烘烤时，地球上的水就会蒸发并凝结成云。

　　What happens if a cloud gets big and thick? It rains. 如果云变得又大又厚时会怎么样呢？就会下雨。

　　Thick clouds are darker than thin clouds. 浓云比薄云的颜色更暗。

　　So, the sky looks grey right before it rains. 因此，下雨之前天空总是灰蒙蒙的。

　　Can we grab a cloud? No, we can't. 我们可以抓住云吗？不能。

　　A cloud consists of water droplets, so we cannot grab a cloud at all. 云是由水滴组成的，我们根本就抓不住它。

　　When you touch it, all you can get is wet hands. 你手碰到它时就湿了。