根据材料内容，选择正确答案。
Look up in the sky. Do you see any clouds? What do they look like? Some clouds are fluffy(绒毛状的) and white, like cotton balls. Others are dark almost black, and make you hold an umbrella. At sunrise and sunset, clouds can look red, purple, or yellow---as if they were colored with crayons. But what are clouds anyway?
You can't reach out and touch the clouds, but you can make your own in a bottle.
You need
* an adult helper
* a huge, empty glass jar
* a metal strainer(过滤器)
* hot water
* ice
Do these
Step1. Have your helper fill the jar with hot water, leave it there for two minutes, then pour out most of the water. Leaving just an inch or two at the bottom of the jar.
Step2. It's your turn, put the strainer over the mouth of the jar, fill the strainer with ice. Watch the jar.
What happened?
A cloud formed in the jar! Some of the hot water at the bottom of the jar turned into hot water vapor(蒸汽). The water vapor rose and met with the cold air coming off the ice. When the water vapor condensed, it formed a cloud. Hot air rises and carries with it lots of water vapor, the higher the air rises, the more it cools down. Soon the cold air can't hold all the water vapor so it starts turning into tiny water drops, and becomes a cloud.
It's two.