Simile - a comparison using the words "like" or "as"
Examples
His arms moved like an octopus.
(comparing the movement of his arms to an octopus)
She was as fast as lightning.
(comparing her quickness to lightning)
Read some Lemony Snicket similes and metaphors!
Metaphor - a comparison that says the thing "is" another thing
Examples
He is a monster.
(an exaggeration to make a point)
She was a tornado moving through the house.
(comparing her to a tornado because of the way she moved)
Idiom - a figure of speech
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Examples
They told the actor to break a leg. (break a leg means good luck)
I need to hit the books.
(hit the books means study)