Is this sentence a simile, personification, metonymy, or synecdoche: "He sat as still as a stone."?
Let's first define these terms and then compare the sentence to them:
Simile - This literary device compares two different things and uses words such as "like" or "as". For example, I could say:
That momma duck is like an army sergeant, keeping her ducklings in a row
He sat as still as a stone
Ok - for sure we have a simile in our sentence.
Personification - This literary device gives something that isn't human, such as an animal or inanimate object, human like traits. For instance, I could say:
The lightening chose the tallest tree in the forest to strike and split in two
The stone sat in contemplation, waiting for its chance to roll down the hill
The sentence in question isn't personification - if it were, the stone would sit like a man and not the other way.
Metonymy - This literary device allows us to refer to a thing (say, "war") but naming a different thing that is closely associated with it (say, "sword") and so we get:
The pen is mightier than the sword - or words are mightier than force and war
And I don't see any way to even begin to use metonymy to approximate our sentence in question. It's not metonymy!
Synecdoche - This literary device is kinda like metonymy but uses a part of the whole to refer to the whole, so I could say:
Nice wheels! - and be referring to a car
Again, our example sentence isn't this.