Sharrow also said that a body of rules for how to write mysteries was developed in the late 1920s. These six rules of fair play are still largely valid today. For instance, one of the rules says that the criminal or guilty party must be mentioned early in the book. No fair introducing a ringer in the last chapter. It also would be considered cheating to have the crime solved through a supernatural source or event.
Supernatural solutions belong to a different genre.
It's also not considered quite fair to have a long scientific explanation for whatever happened. Mystery readers are usually generalists, not scientists. The solution also should not be accidentally stumbled upon by the sleuth and, above all, the sleuth should never be the criminal. These rules also suggest that murder is the only crime worthy of a reader's energy and attention. Sharrow honored the superstars of the mystery genre - Sir Arthur Conan Doyle and Agatha Christie.