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      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 container（容器）

   A metal strainer（滤网）

   Hot water

   Ice

   Do these:

   Step1. Have your helper fill the container 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 container.

   Step2. It's your turn now. Put the strainer over the mouth of the container. Fill the strainer with ice. Watch the container.

What happened?

A cloud formed in the container! Some of the hot water at the bottom of the container turned into the hot water vapor. The water vapor rose and met with cold air coming off the ice. When the water vapor condensed（凝结）,it formed a cloud. Hot air rises ad 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.

气压带、风带的季节移动是形成东亚季风的主要原因。

色盲病是一种遗传病。遗传病不仅会给家庭造成极大不幸，还会降低整个中华民族的人口素质。我们应当采取____ 、____ 、____ 等积极的措施，预防遗传病的发生。