Idioms are a form of figurative language, meaning that they have a different meaning than what they actually sound like. In academic writing, you should avoid idioms, so having a strong knowledge of what they are is important. 

There are many common idioms used in the world today. One common idiom is "It's raining cats and dogs". Obviously, when this phrase is used, cats and dogs are not really falling from the sky (although this is how the term originated, as cats and dogs would sleep on huts made of hay, and the roofs would cave in during strong storms). People use this term when it is raining hard.

Please do the following:

1. Explain what an idiom is and why you believe idioms should not be used in academic writing.
2. List at least one common idiom and what is actually means.