One of the most difficult aspects of learning any foreign language is being able to understand and use a wide range of idioms. The English language has over 15，000 idioms and native speakers use them very frequently，often without realising it themselves. This can make communication with native speakers confusing.
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.