Can you create some variations on the phrase "when pigs fly"?
The phrase when pigs fly is an adynaton – a figure of speech so hyperbolic that it describes an impossibility. The implication of such a phrase is that the circumstances in question (the adynaton, and the circumstances to which the adynaton are being applied) will never occur.
Have you heard of a different phrase to describe an impossibility? If not, create your own!