Idioms have at least two elements even as we have in Chemistry. These two elements combine to form a compound. Two parts are compared to bring out the meaning of an idiom.
Idiom uses the following level of comparisons:
1. METAPHORS, which is comparing something with another sharply or directly as in:
[centre]There are some bad eggs in the police[/centre]
In this expression, you can see the sharp comparison made here. (You can imagine how bad eggs smell)

There are other examples like
[centre]a. The story is a parrot's cry. (parrots are known for their undue alarm)[/centre].

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