This exercise is designed to correct the impression of many students that scientific facts are unchanging. Every scientific fact has a history and its history shapes the form which the fact takes in a textbook. Thus, the history of the octet rule is traced to show that scientific knowledge is not a cast iron set of facts but rather a fluid body of information shaped by the people who use it for specific purposes. The exercise consists of a reading and a set of discussion questions. The questions can be answered by students or could form the basis for a class discussion about the issues raised. It is suggested that the exercise be used after the lesson on the octet rule and Lewis dot structures. (JN)