Inspired by [this question](https://english.stackexchange.com/q/436987) on
English SE, and the [List of idioms of
improbability](https://en.wikipedia.org/wiki/List_of_idioms_of_improbability)
in Wikipedia which does not have an entry for Japanese, I was wondering... Is
there an idiom in Japanese for something that will never happen?
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idioms
Is there an idiom for something that will never happen?
This one is not strictly 'never', but it does indicate very low probability:

瓢箪から駒 (ひょうたん から こま) a horse from a gourd
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This one's in
[jisho.org](http://jisho.org/word/%E7%99%BE%E5%B9%B4%E6%B2%B3%E6%B8%85) with
the definition of "when pigs fly", which is one of the English idioms that
means the same thing:

百年河清【ひゃくねんかせい】を待つ -> Waiting a hundred years for the Yellow River to clear
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