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’．

地震过后往往会伴有泥石流等灾害，泥石流之所以会造成巨大的危害，是因为山石、泥土被雨水从高处冲下来时将____ 能转化为巨大的____ 能。灾害发生后，救灾飞机迅速到达灾区上空，当它在一定高度匀速飞行并投下救灾物资后，飞机的机械能将____ （选填“变大”“变小”或“不变”）。