A simile is a literary device that makes a direct comparison between two things using the words "like" or "as." Bradbury utilizes numerous similes throughout Fahrenheit 451 to add imagery and make his writing more interesting.

After Montag meets his unique, introspective neighbor, Clarisse, he begins to realize that he is not happy with his unfulfilling life. Bradbury utilizes a simile to describe Montag's feelings by writing,

He wore his happiness like a mask and the girl had run off across the lawn with the mask and there was no way of going to knock on her door and ask for it back (5).

At the end of part 2, Montag arrives at the fire station wearing the green bullet in his ear and Captain Beatty attempts to confuse his thoughts regarding literature and knowledge. After Montag listens to Beatty's moving speech, Bradbury once again utilizes a simile to describe Montag's reaction by writing,

Montag sat like a carved white stone (51).

After Montag flees the dystopian city, he enters the wilderness and catches a glimpse of the welcoming campfire that Granger's group is sitting around. Bradbury uses a simile to describe how Montag sees the campfire through the trees by writing,

The fire was gone, then back again, like a winking eye (Bradbury, 68).

After the atomic bomb is dropped and reduces the city to a pile of ashes and rubble, Granger utilizes a simile by commenting,

City looks like a heap of baking-powder. It's gone (Bradbury, 76).

A simile is an indirect comparison.  It usually uses the words “like” or “as” to compare two unlike things.  Clarisse uses two different similes to describe Clarisse’s face.

When Montag first sees Clarisse, he makes an odd comparison.

She had a very thin face like the dial of a small clock seen faintly in a dark room in the middle of a night (part 1).

This simile makes Clarisse seem kind of unusual and exotic, but also fleeting.  She comes into his life rather suddenly and leaves quickly, so this is fitting.

Montag is oddly inspired and unsettled by Clarisse.  She asks him if he is happy, and he has never stopped to think about if he is or not.

How like a mirror, too, her face. Impossible; for how many people did you know that refracted your own light to you? (part 1).

Clarisse is like a mirror because she reflects his true self back to him, and his deepest thoughts.  He is so troubled by his brief encounter with her that he asks himself if he is happy, and he changes his entire approach to life.