Locked room mystery

A Locked room mystery in Crime fiction is a story in which the reader is presented with a puzzle and encouraged to solve it before finishing the story and being told the solution. Its origins, often ascribed to Edgar Allan Poe; this sub-genre of science fiction flourished with the popularity of writers like Agatha Christie.

Typically, A "locked room" in this narrow meaning of the word -- also referred to as a "hermetically sealed chamber" -- is a room in which a murder is committed. There are a limited number of suspects, some of them possibly even without a watertight alibi. But on closer inspection, it turns out that no one could possibly have perpetrated the murder, because at the time the murder was committed, there was definitely no way of entering and/or leaving the room unseen or without leaving a trace.

The prima facie impression, almost invariably would be that the perpetrator has vanished into thin air.

The following is an example scenario in such novels

The only door, locked from the inside, has to be forced open (and the position of the body clearly suggests that the victim could not have locked it after being struck down by the killer).
There is no fireplace or chimney through which the murderer might have escaped.
The only window is barred from the inside, or there is virgin snow on the window sill outside.
There is no secret passage leading to, or trapdoor anywhere in, the room.
The murder weapon is nowhere to be found although the victim has clearly been poisoned, stabbed, shot or strangled (with the cause of death established beyond doubt in an autopsy some time later).
If the victim has been electrocuted, no live wires can be found anywhere near the corpse; if they have been shot, no one within usual hearing distance remembers hearing a report.

These "facts" strain the interest of the reader, and builds up a palpitating curiosity to crack the truth, and explains the huge popularity of such novels among common people. To appeal to this sense of curious suspense,in many locked room mysteries, plausibility was neglected in favour of ingenuity and maximum reader involvement. Among avid readers, heated discussions ensued after the publication of a particular novel on whether it is really possible to commit the perfect murder the way it is described in the book.

These may be some example loopholes, that a reader may find:

If the victim is found stabbed in a locked room on one of the upper floors of a building, no murder weapon is found and a window giving onto a backyard has been left open, could they have been killed by a professional knife-thrower from the building across the yard by means of a knife to which a long cord was attached? (This is a variation on the "dagger with wings" idea.)
Can eye-witnesses be deluded into thinking they have seen a particular person enter or leave a room when in fact what they saw was just an image in a mirror?
Can you gain access to a house by posing as someone else, wearing clothes made of paper, and then get rid of them -- as they would be evidence if you did not -- by burning them in the open fire? And are there really murder weapons that dissolve over time one way or another?

The Hollow Man is considered to be the most famous locked mystery novel of all times. It gives an explicatory recipe for crime writers. Chapter 17 of the book, consists of a theoretical digression entitled "The Locked-Room Lecture". In it, Dr Fell (the detective) gives an extensive explanation of how the murderer is able to deceive everyone else (at least until the riddle is finally solved). How, for example, Fell asks, can the perpetrator create the impression of a hermetically sealed chamber when in fact it is not? What means are there of tampering with a door so that it seems to be locked on the inside? This is just one of the answers -- and, as it happens, a most simple one -- given by Fell:

[...] An illusion, simple but effective. The murderer, after committing his crime, has locked the door from the outside and kept the key. It is assumed, however, that the key is still in the lock on the inside. The murderer, who is first to raise a scare and find the body, smashes the upper glass panel of the door, puts his hand through with the key concealed in it, and finds the key in the lock inside, by which he opens the door. This device has also been used with the breaking of a panel out of an ordinary wooden door.

Many authors tried their hand at new and far-fetched yet eventually plausible locked room scenarios, with one of the underlying principles always being that supernatural powers or any form of magic must be ruled out from the start. American writer Anna Katharine Green (1846 - 1935) wrote Initials Only (1911), Margery Allingham (1904 - 1966) exploited the same motif in Flowers for the Judge (1936), and many more joined the ranks.