A simile is a figure of speech consisting of a comparison using like or as. Well-chosen ones can be used to enliven writing or as an alternative to description using adjectives, for example. Some refer to a universal, or nearly so, quality or attribute of the object of comparison.
Examples include the following:
As white as snow
As old as the hills
It is also possible for these figures of speech to refer to a characteristic that is possible but not necessary:
As high as a kite
Kites are not always being flown, so they are not always high, but flying is what kites are made for and being "up in the sky" is closely associated with them. Even though they are not quite as obvious, such similes are readily understood.
Similes can also be sarcastic:
As clear as mud.
This is not intended to be understood in a straightforward way; it means, in fact, the opposite of what it says.
Some comparisons expressed this way tend toward the figurative:
As dead as a doornail
has been explained in several different ways: as deriving from the practice of bending back doornails so that they could not be salvaged, or possibly from the use of doornails as early doorknockers, dead to the "abuse" they received as they were knocked. In any case, doornails are not literally dead, having never actually been alive.
Another figurative use is the following:
As dead as a dodo
because, technically, dodos aren’t dead, but extinct.
Alliteration may play a role in these as well, because a pattern can certainly be seen:
As clear as crystal
As dry as dust
As busy as a bee
As proud as a peacock
As pleased as Punch
As large as life
As pretty as a picture
As cool as a cucumber
As dull as dishwater
As hot as hell
As fit as a fiddle
As slow as a snail
Similes start off as fresh, interesting language, but like any other phrase, when too often repeated, they become clichés. Most of the color comparisons and animal comparisons have lost their verve, for example. Instead of brightening up language, they now make it boring. It’s not enough to sprinkle one’s language with these figures of speech to enliven it — it’s also necessary to choose well.