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 ". 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'. It is most probable that idioms will be seen  _  .
in a paper in an preface in a notice in online chatting