in this phrase thus erring once more against the rigidity of structure which is so characteristic of phraseological units.
Yet again, as in the case of restriction in introducing additional components, there are exceptions to the rule, and these are probably even more numerous.
One can build a castle in the air, but also castles. A shameful or dangerous family secret is picturesquely described as a skeleton in the cupboard, the first substantive component being frequently and easily used in the plural form, as in: I'm sure they have skeletons in every cupboard! A black sheep is a disreputable member of a family who, in especially serious cases, may be described as the blackest sheep of the family.
Consider the following examples of proverbs:
We never know the value of water till the well is dry.
You can take the horse to the water, but you cannot make him drink.
Those who live in glass houses shouldn't throw stones.
Even these few examples clearly show that proverbs are different from those phraseological units which have been discussed above. The first distinctive feature that strikes one is the obvious structural dissimilarity. Phraseological units, as we have seen, are a kind of ready-made blocks which fit into the structure of a sentence performing a certain syntactical function, more or less as words do. E. g. George liked her for she never put on airs (predicate). Big bugs like him care nothing about small fry like ourselves, (a) subject, b) prepositional object).
Proverbs, if viewed in their structural aspect, are sentences, and so cannot be used in the way in which phraseological units are used in the above examples.