What is an idiom? An idiom is a group of words which, when used together, has a different meaning from the one which the individual words have. For example, “---How do you know that Sid and Nancy have separated ?” “---I heard it on the grapevine(葡萄藤)”.
Of course, the second speaker does not mean he heard the news about John by putting his ear to a grapevine! He is conveying the idea visually of information spreading around a widespread network, similar to a grapevine. We use idioms to express something that other words do not express as clearly or as cleverly. We often use an image or symbol to describe something as clearly as possible and thus make our point as effectively as possible. For example, "in a nutshell" suggests the idea of having all the information contained within very few words. Idioms tend to be informal and are best used in spoken rather than written English.
One of the best ways to learn an idiom is by looking at the context in which it is used. This can be done by concentrating on the rest of the sentence and try to guess the meaning. Many idioms are not that difficult to understand when considered in their context. For example: We are going to have a surprise party for Tom tomorrow. It's a secret so please don't let the cat out of the bag. 
　'Let the cat out of the bag' is an idiom. Imagine you don't know what this idiom means; by looking at the words preceding, it should be easy to guess that the speaker does not want you to tell Tom about the surprise party. Therefore, 'let the cat out of the bag' must mean something like' reveal a secret' or 'tell a secret'.
Which of the following sentences is NOT similar to  “I heard it on the grapevine. ” in figures of speech(在修辞方面)？

A．The exam was a piece of cake.

B．You will be paid under the table.

C．You eat with that mouth?

D．It rained cats and dogs yesterday.

“I was feeling under the weather so I went to see a doctor.” The underlined idiom  means____.

A．blue

B．homesick

C．unhappy

D．ill

It is most probable that idioms will be seen______.

A．in a paper

B．in an preface(序言)

C．in a notice

D．in online chatting

Which of the following statements is true according to the passage?

A．Other words can express something as vividly as idioms.

B．Written English with spoken English does not use idioms at all.

C．There are plant, food, animal idioms used to express something.

D．Contexts play not a bit important part in guessing the meaning of idioms.