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I am looking for good way of translating my native language way of saying something is impossible and will never be possible, i.e.,
Kan lungtat a par hunah le which literally means 'When our grindstone comes into blossom'. The grindstone which is used for sharpening knife can never sprout and grow tall and come into bloom. For example, So when someone asks for the impossible, we respond 'When our grindstone blooms' for emphasizing the impossibility. Is there an equivalent phrase in English?