Idioms often don’t make sense.
Example: When pigs fly means it will probably never happen.

They are not understood unless you are familiar with their meaning.
Example: Kick the bucket means to die
Someone unfamiliar with this idiom would not have understood the humor in the death scene of Jimmy Duranty in It’s a Mad, Mad, Mad, Mad World as in this short clip.

Idioms differ from country to country, nationality, region and ethnicity. For this reason, generally their use is not acceptable in formal speech.

Someone from France might know that something will never happen when chickens have teeth, but not when pigs fly.

A German might know how to tie a bear to someone.

It would be dreadful to have your cheeks fall off while eating unless you were Japanese.

You can find a lot of idioms online, maybe also learn some new. And if you haven’t seen It’s a Mad, Mad, Mad, Mad World check it out. It’s a cl;assic and packed with an allstar cast, it’s from 1963.