Quiz Example 4-Hyperbole or Idiom?

Identify whether the sentence is using a hyperbole or an idiom and explain.  

Example:  She served up mountains of ice cream.    

The sentence is using a hyperbole.  Mountains of ice cream is an exaggeration to let us know she serves a lot of ice cream.  

Example:  I only see my cousin once in a blue moon.  

The sentence is using an idiom.  Once in a blue moon is a saying that means not very often.