The impossible clue and what it means
The supernatural mystery's most distinctive structural element is the impossible clue: the evidence that cannot be explained by any natural means and that therefore forces the investigator to consider supernatural explanations they would normally exclude. Writing the impossible clue requires understanding what makes it specifically impossible — not generally strange, but specifically beyond the range of what the available suspects could have produced — and ensuring that its impossibility is established clearly enough for the reader to understand why it matters. The impossible clue should narrow rather than expand the possibilities: it should rule out all natural explanations convincingly enough that the supernatural explanation is the only one remaining, not simply be weird enough to suggest that something unusual happened.