To bring two different things together with an explicit word, such as "like", "just as" or "seems". Unlike metaphor, comparison leaves visible that they are two things.
In Madame Bovary
... whose mute ugliness has depths of expression, like the face of an imbecile
The sentence links the ugliness of the cap to the "face of an imbecile," comparing an object to a human expression. The "like" leaves the comparison visible and makes the cap seem already to say something about the boy who wears it.
Charles's conversation was as flat as a sidewalk, and everyone's ideas paraded through it in their everyday clothes
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her life was cold as an attic whose skylight faces north, and boredom, a silent spider, spun its web in the shadow at every corner of her heart
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had torn a hole in her life, like one of those great crevasses that a storm sometimes tears in the mountains in a single night
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like sailors in distress [...] she cast desperate eyes over the solitude of her life, searching far off for some white sail in the mists of the horizon
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the little cardboard berries crackled, the wire threads twisted, the braid melted; and the paper corollas, shriveled, swaying along the plaque like black butterflies, finally flew up the chimney
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Poor little woman! I'll bet she longs for love like a carp longs for water on the kitchen table! With three words of flattery, I'm sure she would even adore me!
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Rodolphe pressed her hand and felt it very warm and trembling, like a captive dove that wants to take flight again
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What an abundance of illusions! Now she had none left! She had spent them all on the adventures of her soul, in every successive condition, in virginity, in marriage and in love, losing them continually throughout her life, like a traveler who leaves part of his wealth at every inn along the way.
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human speech is like a cracked kettle on which we bang out melodies to make the bears dance, when what we meant was to move the stars to pity
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a carriage with its blinds drawn keeps appearing like this, more closed than a tomb and shaken like a ship
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her life was nothing but a collection of lies, in which she wrapped her love as in veils, to hide it
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and her life, down to its most intimate corners, was, like a corpse undergoing an autopsy, exposed wide open to the gazes of those three men.
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