最近，我发现了一道既陈旧、又新鲜的物理习题．题目大意是：“一人家有前后二门，现要求装一门铃，前后门都要有开关，请你帮助设计一个电路图．为了区别是前门来人还是后门来客，电路图该如何改进？” 我说它陈旧，是因为它源于老的初中物理课本．说它新鲜，是因为连续二年我 Recently, I found an old and fresh physics problem. The main idea is: “A person has two doors before and after, now requires a doorbell, front and rear doors must have a switch, please help design a circuit diagram. In order to distinguish between the front door or the back door visitor, how to improve the circuit diagram?” I said It is outdated because it originated in old junior high school physics textbooks. It’s fresh because it’s been two years in a row.