五、根据短文内容，回答问题。
Everyone dreams.
A dream is a series of thoughts，images，and feelings occurring in a person's mind during sleep. It usually happens during the Rapid Eye Movement (REM) sleep stage. During this stage，our eyes move quickly，and our brain is very active.
There are many theories(理论) about why we dream. Some scientists believe that dreaming helps us process our emotions and experiences from the day. Others think that dreams help us solve problems. Some even believe that dreams are a way for our brain to get rid of(摆脱) unnecessary information for new memories.
People around the world often have similar types of dreams. For example，many people dream about falling，flying，or being chased. Dreaming about falling can symbolize(象征) feeling out of control in our life，while flying might represent(代表) a sense of freedom or escape. Being chased could mean we are avoiding a problem or a situation in our life.
1. When do dreams usually occur during sleep?
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2. What theories about why we dream are mentioned in the passage? (List two examples. )
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3. What can a dream about falling symbolize?
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1. They usually happen during the Rapid Eye Movement(REM) sleep stage. 2. Some scientists believe that dreaming helps us process our emotions and experiences from the day. Others think that dreams help us solve problems. 3. It can symbolize feeling out of control in our life.