<human>:  What's the difference between a metaphor and an idiom?
<bot>: A metaphor is a figure of speech that makes a comparison between two things that are unrelated and suggests a similarity, while an idiom is an expression with a figurative meaning that is different from the literal meaning of its words. For example, "raining cats and dogs," is an idiom that means it is raining heavily and not that literal cats and dogs are falling from the sky.