There once was a famous professor, who always wrote the number 2 and 4 on the blackboard at the first course to his new students, and asked the students what is the answer.
一位非常著名的教授，每次给新生上第一堂课时，总是在黑板上写下“2”和“4”两个数字，然后转过身问学生：“答案是什么?”

Every time when he asked this question, some of his students said 6 is the key while some others hold that 2 is more proper and some of them chose 8 as the answer, moreover, there were still others who were just sitting at their seats without a word.
　 有的同学说是“6”，有的同学说是“2”，还有的说是“8”。其他的一些人感觉无聊，坐在那里一言不发。

But at the time that the students were discussing the issues, the professor always sighed and said to the students “It is no use for you to debate because all of you did not focused the key point: what exactly the question is asked about? Do those two numbers were being to be added, subtracted or be acquired to do some other calculations? Without what the question is exactly about, how can you make a right respond”.
　 就在大家议论纷纷的时候，教授叹着气对学生们说：“你们争论是没有用的，因为你们都没有注意到一个关键的问题：这个问题到底在问什么?是加是减，是乘是除?还是其他什么?不知道问题的所在，怎样能得出正确的答案？”

(中文来自：小故事网 翻译:Ellison)