这段话的topic sentence很明显-“Hobbies are important for many reasons”，之后的支持句从三方面阐述hobby的重要性(first， second， third)，但段中这句话"A person can also meet other people by going the school"与hobby重要的原因没有关系，因此削弱了整个段落的连贯性与统一性，应该被去掉。
3.Add details. To write a more fully developed paragraph， you need to add details to your supporting ideas. Your details can be facts， examples， personal experiences， or descriptions.
(需要用多种方式提供细节展开段落)
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 SpaceMuseum， 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 andSpaceMuseum， 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 andSpaceMuseum， 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.
