American people like to say "Thank you " or say something kind when others help them．People of many countries do so，too．It is a very good habit．
    You should say"Thank you'when someone passes you the salt on the table，when someone walking ahead of you keeps the door open for you and when someone says you always do your work well or you have a nice thing or your city is very beautiful．“Thank you”is used not only between friends，but also between parents and children，brothers and sisters．
    “Excuse me”is another short sentence they use．When you hear someone says“Excuse me”behind you，you know that someone wants to walk past you without touching you．It’s not polite to break(打断)others when they are talking．If you want to speak to one of them。say‘Excuse me’first，and then begin talking．You should also do so when you begin to cough or make any noise before others．
Let‘s learn to say‘Thank you’and‘Excuse me’．

超声波及其应用

    人能够听到声音的频率范围从20Hz到20000Hz。低于20Hz的声音叫次声波，高于20000Hz的声音叫超声波。

    超声波具有许多奇异特性：空化效应──超声波能在水中产生气泡，气泡爆破时释放出高能量，产生强冲击力的微小水柱，它不断冲击物件的表面，使物件表面及缝隙中的污垢迅速剥落，从而达到净化物件表面的目的。传播特性──它的波长短，在均匀介质中能够定向直线传播，根据这一特性可以进行超声探伤、测厚、测距、医学诊断等。

假设你是思考，你的美国笔友Lucy来信想了解曹冲称象的故事。请根据如下图片及要点提示用英语写一封80词以上的回信给她。

内容要点：1.回信目的

          2.故事简介

          3.故事给你的启示

注意：1.信中不允许出现与考生本人相关的真实姓名和校名等信息。

     2. 信的开头已给出，不计入总次数。

     3. 参考词汇：scale n.称; calculate v.计算

Dear Lucy,                                                                                                                    

I learn from your letter that…