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It often use the word is, was , are ,were. It should compare two things.
For instance:-He swan in the sea of diamonds.
There they are comparing between sea and diamonds.

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I you are convinced please as best aswer.
Instead of I it is If. Typing error.
Let me explain this with the help of some examples:-
Metaphor is indirect comparision between two things. If there is any comparision between two things without "like, as well as, etc there is metaphor.

   eg- Camels are ships of sand.
Here there is comparision between camels and ships but indirectly.