A simile is a figure of speech where one thing is compared with another thing of a different kind, used to make a description more emphatic or important. An example could be as brave as a lion or like a lamb to the slaughter.
Typically, similes are using comparisons using like or as to compare two things.
Easy as shooting fish in a barrel.
Cool as a cucumber
Blind as a bat
Light as a feather
Like watching paint dry
Works like a charm
Old as the hills
Pretty as a picture
Hurts like the devil
Strong as an ox
Fight like cats and dogs
Solid as the ground we stand on.
Nutty as a fruitcake.
Goes down like a lead balloon.
Different as night and day.
Thin as a rake.
Slept like a log.
Fits like a glove.
Stand out like a sore thumb.
Deep as the ocean.
Dry as a bone.
From this we can see that a simile directly compares two things to be the same as one another or like one another.
When should we use similes in spoken English? Essentially when we want to compare something unknown with something that is more familiar.
Similes often make use of a hyperbole, or exaggeration:
He ran as fast as lightning. Obviously, no one can run at the speed of light, but the exaggeration gives the idea that the person was running as fast as possible. The use brightens up the language and gives more emphasis to the topic in hand.
Reasons to use similes;
to say something in a new way
to help the listener form a picture in their mind
to make something interesting and descriptive
for emphasis or stress
to make something clear or easy to understand