Expressions "As cool as a cucumber", "As fresh as a daisy". "As red as lobster", "AS fit as fiddle" come to express the quialities of things, to better understand them in comparance with familiar feelings. As to the "cucumber", this expression means something that has relatively low temperature, relatively cold, as the fresh cucumber.

There's also a kind of poetry which keeps expressions like this popular - in this case, alliteration. Vica's other example of 'Fit as a Fiddle' is a good one, as well as "Good as Gold". Others which rhyme but make no sense still stay popular, eg. "drunk as a skunk". So there's not always a perfectly logical explanation for these phrases... they just 'sound right'. Then there's other expressions which don't rhyme, alliterate OR make sense, but still 'sound right', eg. "pissed as a newt"! (=very drunk).

idk..