阅读短文，下列问题
Mary did not understand such sentences as “She is in blue today.” “You are yellow.” “He has a green thumb.（大拇指）” “He has told a white lie.” and soon. And she went to her teacher for help.
Mary: Mrs. Smith, there is a color in each of these sentences. What do they mean?
Mrs. Smith: In everyday English, Mary, blue sometimes means sad. Yellow means afraid. A person with a green thumb grows plants well. And a white lie is not a bad one.
Mary: Would you give me an example for a white lie?
Mrs. Smith: Certainly. Now I give you some cake. In fact you don't like it, but you won't say it. Instead, you say,“No, thanks, I'm hungry.” That's a white lie.
Mary: Oh, I see. Thank you very much.