For effective teaching ，subject matter knowledge is not a question of being a walking encyclopedia.Effective teachers not only know their subjects,but they can also communicate their knowledge to students.The celebrated high school math teacher Jaime Escalante taught the concept of positive and negative numbers to students in a Los Angeles barrio by explaining that when you dig a hole,you might call the pile of dirt +1，the hole-1.What do you get when you put the dirt back in the hole?Zero.Escalante's ability to relate the abstract concept of positive and negative numbers to his students ' experience is one example of how the ability to communicate knowledge goes far beyond simply konwing the facts.
为了有效的教，学科知识不是一个行走的百科全书式的问题。
效率高的教师，不仅自己知道学科知识，而且还能把知识传递给把知识传递给学生。
一所著名中学的数学老师在教授正负数概念的时候，他是这样解释的，当他挖挖一个洞的时候，他把挖出的一部分成为正一，这个洞称为负一。
那么把土放回去的时候会发生什么呢？零。他的把抽象的正负数概念和学生的经历联系起来的能力是一个很好的实例，用来说明想学生传递知识的能力比知识本身重要。
