If you eat in a restaurant, you absolutely must not forget to leave a tip. And if someone holds a door for you while you are hurrying about your business, you can't just barrel on past, lost in thoughts of your own affairs, but must pause and offer some words of thanks.
所以在餐廳用餐完畢，可別忘了留下小費；趕路時，如果前面的行人，替你開地鐵的大門，也別只顧慌張地往前衝，要先說謝謝、謝謝。