until metamorphosing into a fossilised term. This collocation of words redefines each component word in the word-group and becomes an idiomatic expression. Idioms usually do not translate well; in some cases, when an idiom is translated directly word-for-word into another language, either its meaning is changed or it is meaningless. When two or three words are conventionally used together in a particular sequence, they form an irreversible binomial. For example, a person may be left "high and dry", but never "dry and high". Not all irreversible binomials are idioms, however: "chips and dip" is irreversible, but its meaning is straightforwardly derived from its components. Mobility Idioms possess varying degrees of mobility. Whereas some idioms are used only in a routine form, others can undergo syntactic modifications such as passivization, raising constructions, and clefting, demonstrating separable constituencies within the idiom. Mobile idioms, allowing such movement, maintain their idiomatic meaning where fixed idioms do not: Mobile I spilled the beans on our project. → The beans were spilled on our project. Fixed The old man kicked the bucket. → The bucket was kicked (by the old man). Many fixed idioms lack semantic composition, meaning that the idiom contains the semantic role of a verb, but not of any object. This is true of kick the bucket, which means die. By contrast, the semantically composite idiom spill the beans, meaning reveal a secret, contains both a semantic verb and object, reveal and secret. Semantically composite idioms have a syntactic similarity between their surface and semantic forms. The types of movement allowed for certain idioms also relate to the degree to which the literal reading of the idiom has a connection to its idiomatic meaning. This is referred to as motivation or transparency. While most idioms that do not display semantic composition generally do not allow non-adjectival modification, those that are also motivated allow lexical substitution. For example, oil the wheels and grease the wheels allow variation for nouns that elicit a similar literal meaning. These types of changes can occur only when speakers can easily recognize a connection between what the idiom is meant to express and its literal meaning, thus an idiom like kick the bucket cannot occur as kick the pot. From the perspective of dependency grammar, idioms are represented as a catena which cannot be interrupted by non-idiomatic content. Although syntactic modifications introduce disruptions to the idiomatic structure, this continuity