the Rubicon — the Rubicon is crossed (passed); to shed crocodile tears — crocodile tears are shed. Hence, if one accepts nomination as a criterion of referring or not referring this or that unit to phraseology, one is faced with the absurd conclusion that such word-groups, when with verbs in the Active Voice, are phraseological units and belong to the system of the language, and when with verbs in the Passive Voice, are non-phraseological word-groups and do not belong to the system of the language. 
It may be added, as one more argument in support of this concept, that there does not seem to exist any rigid or permanent border-line between proverbs and phraseological units as the latter rather frequently originate from the former.
So, the phraseological unit the last straw originated from the proverb The last straw breaks the camel's back, the phraseological unit birds of a feather from the proverb Birds of a feather flock together, the phraseological unit to catch at a straw (straws) from A drowning man catches at straws.
What is more, some of the proverbs are easily transformed into phraseological units. E. g. Don't put all your eggs in one basket > to put all one's eggs in one basket; don't cast pearls before swine > to cast pearls before swine.
I. Consider your answers to the following.
What do we mean when we say that an idiom has a "double" meaning?
Why is it very important to use idioms with care? Should foreign-language students use them? Give reasons for your answer.
The term "phraseological unit" is used by most Russian scholars. What other terms are used to de scribe the same word-groups?