The so-called idiom is a fixed phrase formed by long-term use and tempering in language. It is a language unit larger than a word, and its grammatical function is equivalent to a word. Idioms are mostly composed of four words.

Features: ① The structure is relatively stereotyped. (2) the integrity of meaning. ③ Habituality of time and space. ④ Historicity of formation. ⑤ Nationality of content and form.

Source: ① Historical stories. 2 fables. (3) Myths or other legends. 4 classic literary works.

Idioms are fixed phrases or sentences expressing general concepts, which are mostly composed of four words. For example, "castles in the air", "famous", "according to you", "colorful" and "happy" are all four-character idioms. Idioms with less than four words, such as "knocking at the door", "unwarranted" and "taking for granted", and idioms with more than four words, such as "peaches and plums are all over the world", "real gold is not afraid of fire", "a leopard cannot change his spots" and "only state officials are allowed to set fires, but people are not allowed to light them", account for an absolute minority.