根据第3条的原则，我们来看下面这个段落：
The Smithsonian Institution is worth visiting for a number of reasons. The Smithsonian Institution comprises various museums that offer something for everyone. These museums include the National Museum of History and Technology, the National Aeronautics and Space Museum, the National Collection of Fine Arts, the National Museum of Natural History, and several others. A person can do more than just look at the exhibits. For example, in the insect zoo at the National Museum of Natural History, anyone who so desires can handle some of the exhibits. The museums provide unforgettable experiences. In climbing through the Skylab exhibit at the National Aeronautics and Space Museum, I was able to imagine what it would be like to be an astronaut in space. Movies shown at regular intervals aid in building an appreciation of our world. In the National Aeronautics and Space Museum, there is a theater that has a large screen. When the movie is shown, it gives the viewer the feeling that he or she is in the movie itself, either floating above the Earth in a hot-air balloon or hang gliding over cliffs.
这是一个成功的主题段，请注意这段话段落细节展开的几种形式-提供事实、举例、个人经历和描述。我们把这个段落拆分后进行分析：
Topic sentence(主题句)
The Smithsonian Institution is worth visiting for a number of reasons.
Supporting idea 1(支持理由1)
The Smithsonian Institution comprises various museums that offer something for everyone.
Details – facts(细节展开-提供事实)
These museums include the National Museum of History and Technology, the National Aeronautics and Space Museum, the National Collection of Fine Arts, the National Museum of Natural History, and several others.
Supporting idea 2(支持理由2)
A person can do more than just look at the exhibits.
Details – examples(细节展开-举例)
For example, in the insect zoo at the National Museum of Natural History, anyone who so desires can handle some of the exhibits.
Supporting idea 3(支持理由3)
The museums provide unforgettable experiences.
Details – personal experience(细节展开-个人经历)
In climbing through the Skylab exhibit at the National Aeronautics and Space Museum, I was able to imagine what it would be like to be an astronaut in space.
Supporting idea 4(支持理由4)
Movies shown at regular intervals aid in building an appreciation of our world.
Details – description(细节展开-描述)
In the National Aeronautics and Space Museum, there is a theater that has a large screen. When the movie is shown, it gives the viewer the feeling that he or she is in the movie itself, either floating above the Earth in a hot-air balloon or hang gliding over cliffs.
