American people like to say "Thank you" whenever others help them. People of many other 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, when someone says you have done your work well, or you have bought 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, husbands and wives. "Excuse me" is another short sentence they use. When you hear someone say "Excuse me" behind you, you know that somebody wants to walk past you without touching you. It's not polite to interrupt  others while 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 want to cough or make any unpleasant noise before others. Let's all learn to say "Thank you" and "Excuse me". You should learn to say "Thank you" when   _  .
you say something kind to others you help others you are helped by others you need others to help