Something extremely unlikely or impossible to occur; equivalent to 'hell freezes over' or 'when pigs fly'.

if you live a life of crime, sooner or later you'll pay for it

to emerge; to pop up; to spring forth; to appear from nowhere

to put someone or something to the test to show their true ability or quality; the proof of the pudding is in the eating

to publish or appear in print (in a newspaper, magazine, or other publication)

to appear, emerge, or occur suddenly and unexpectedly