to teach a fish how to swim

你有听说过不会游泳的鱼吗？你有听说过鱼因不会游泳而被淹死的事吗？如果谁有这样的担忧，我想和那个被人嘲笑了几百年的担心天会塌下来的杞国人就没有什么差别了，必定会成为人们茶余饭后的笑料。

作为一种本能，鱼儿天生就是会游泳的，完全适应水底生活，如果有人想教鱼儿how to swim，这和在鲁班门前卖弄使斧头的功夫，在孔老夫子面前卖弄写文章的本领又有什么差异呢？

因此，to teach a fish how to swim 的含义就是〖班门弄斧〗,〖在孔夫子面前卖文章〗。英语中类似的表达还有：teach a dog to chase rabbits; show the President where the White House is; teach the Pope how to pray; use Chinese maxims in front of Confucius。

TO STAB SOMEONE IN THE BACK意为『暗箭伤人』，此乃小人之所作为，其行为本就难以提防，若是被自己的朋友从后头捅了一刀，那就更是令人措手不及，悲愤填膺了。

传说凯撒大帝(Julius Caesar)在罗马元老院里被人谋杀濒死之前，骇然发现亲信好友布鲁托斯(Brutus)竟也是叛徒之一。惊慌之中他喊出了生命中最后一句控诉："Brutus, you too!" 真是道尽了满心的凄楚与无奈。例如：

A: You look awful. Are you sick?
B: Yes, sick at heart.
A: Why? What happened?
B: I told my friend about a job I was going to apply for. He applied for it himself-and got it.
A: Too bad. But you'll find another job.
B: I don't care about the job. I'm upset because I've been stabbed in the back by a friend.