15．1下定义法
下定义（definition）就是解释某事物是什么，不是什么。写下定义段落时，通常把概念或要下定义的事物置于一个大的类别中，然后解释该事物与同一类别的其它事物有何不同。下定义法多用作推展段落的辅助方法。
示范段落15－1
A pencil is an instrument for writing on paper．It is of cylindrical shape and can be held between the first two fingers of the hand．It is about 6 inches long and 1／4 inch wide．At the end there is usually an eraser attached to the pencil by a surrounding metal band．Lead is the material which is used in making the marks on the paper，and a sharpener is needed to keep the pencil in good working order．Pencils are made of various kinds of wood．
15．2示范段落分析
主题句：A pencil is an instrument for writing on paper．细节：shape，material
15．3关于如何写下定义段落
1）以正式的定义开始
一个正式的定义段落通常把要定义的术语置于一个概类中，然后指出该术语与其它同类别事物有什么不同。在示范段落15－1中，要定义的术语是"pencil"，属于概类"instrument"。
2）用描述、举例或比较对比方法扩充定义
在正式的定义之后，必须更详细地解释该术语的意思，在示范段落15－1中用的是描述法，在示范段落13－1中，扩充"courage"概念用的是举例法。比较对比法也可以用于扩充定义，对比法陈述该事物不是什么，比较法陈述该事物象什么。
Exercise 15-1
Directions：Read the following paragraph，point out the topic sentence and find out what method（s）are used to expand the defintion．
Chemistry is that branch of science which has the task of investigating the materials out of which the universe is made．It is not concerned with the forms into which they may be fashioned．Such objects as chairs，tables，vases，bottles，or wires are of no significance in chemistry；but such substances as glass，wool，iron，sulfur，and clay，as the materials out of which they are made，are what it studies．Chemistry is concerned not only with the composition of such substances，but also with their inner structure．
Topic Sentence：____________________________________________
Method（s） used：_______________________________________

Exercise 15－2
Directions：The definition of"an ice－box"can be，"An ice－box is an appliance for storing food at low temperature．"Now use the method of description to expand the definition．
An ice－box is an appliance for storing food at low temperature．
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Exercise 15-3
Directions：Give the defintion of friendship and then use an example to expand your definition．
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