The only sure thing about luck is that it will change.

The remark treats “luck” not as a stable possession but as a fluctuating condition. Its point is both consoling and cautionary: good fortune is temporary, so complacency is foolish; bad fortune is also temporary, so despair is premature. By calling change the “only sure thing,” the epigram turns luck into a synonym for volatility and reminds the reader to plan and act without assuming that circumstances—favorable or unfavorable—will last. In Mizner’s characteristic tone, it’s a pragmatic, slightly cynical counsel to keep perspective and remain adaptable.