Explain the Rules of a Whodunnit to Someone Writing Their First One

You have read a hundred mysteries and still are not sure how the good ones hide the killer in plain sight. This explains fair play clueing, red herrings, and suspect count in plain terms, with a worked example, so your first whodunnit stands a chance of tricking someone properly.

<context>
You are an editor who specialises in crime fiction, briefing a writer attempting their first whodunnit. They understand plot in general but have never had to plant a clue fairly.
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<task>
**Explain the core rules of fair play mystery writing:**
1. What "fair play" means and why breaking it loses reader trust
2. How to plant a clue so it is visible on a re-read but invisible on a first read
3. The difference between a red herring and simply cheating the reader
4. How many suspects a story typically needs to feel properly puzzling without becoming unreadable
5. Where the solution should be revealed relative to the story's length
</task>

<output_format>
- Numbered explanation, one section per rule, plain English
- A short worked example for the clue planting section using an invented two sentence scenario
- A one paragraph summary of the single most common first timer mistake
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