At the end of each performance of the Agatha Christie play "The Mousetrap," the person revealed to be the murderer steps forward and tells the audience to "keep the secret of whodunit locked in your heart."
Even after 58 continuous years of performances in the West End of London, the play's twist ending has been largely preserved by reviewers, guidebook writers and the great bulk of the estimated 10 million people who have seen the play.
There is one notable exception: Wikipedia. The encyclopedia's article about the play succinctly summarizes its two acts and then, in a single sentence, even more succinctly explains who the killer is.
That shocked Christie's grandson, Matthew Prichard, who told the British newspapers late last month that he was dismayed to learn that Wikipedia could not keep a secret.
"I don't pretend to be an expert on Wikipedia or modern technology," he told The Independent. "All I can tell you is that from the point of view of the theatre going public, I think it does spoil the enjoyment of those going to have an entertaining evening at the theatre -- one part of which is to guess who the murderer is."
In an age of information overload, it is getting harder and harder to preserve mystery and surprise in entertainment -- that can mean Olympics results being reported via Twitter before they are shown at night on television, or TV cliffhangers that are scooped by people on the set or Web sites that offer the endings of celebrated movies and plays.
After all, recent and even old films and plays may as well be brand new to the people who have never seen them and might not know that Darth Vader is really Luke Skywalker's father (oops -- sorry about that).
"My grandmother always got upset if the plots of her books or plays were revealed in reviews," said Mr. Prichard, who was given the rights to the play as a present in 1952 for his ninth birthday, "and I don't think this is any different."
Still, "Mousetrap" has had nearly six decades for its secret to leak out. The documentary "Catfish" had its surprise ending revealed by Wikipedia before it even played in movie theatres -- the brief plot summary and its spoiler were based on a screening at this year's Sundance Film Festival.
"It's hard to argue that there is an intellectual or academic reason for getting deeply into the secrets of a movie that the vast majority of the public has not had access to," said Andrew Jarecki, a producer of "Catfish," whose 2003 documentary, "Capturing the Friedmans," also contained crucial plot twists. At a minimum, Mr. Jarecki said, Wikipedia should offer a so-called spoiler alert to warn readers that "you are about to enter a section likely to harm the experience of the movie for you."
There is little likelihood of that happening. Wikipedia considers itself a comprehensive reference work and thus doesn't censor facts or even warn readers that a spoiler is on the way.
As Jay Walsh, a spokesman for the foundation that operates Wikipedia, put it: "Generally it appears most Wikipedians support the notion that encyclopedias are often exhaustive when it comes to facts, and someone searching for an article about a story should be prepared to encounter a summary of the plot."
Mr. Jarecki himself reported that he knew a lot of Wikipedia users sympathetic to his film, "who have tried to edit that description, who are registered users, and have been unsuccessful."
Movies, plays and books may be caught in the worst bind.
"There is a distinct difference between spoiling something, and writing about something after it happens," says Carlton Cuse, an executive producer of the "Lost" TV series, whose episodes are individually summarized at Wikipedia. " 'The Mousetrap' is a work that exists in both spaces."
Mr Cuse said he had no problem with Wikipedia printing the plot of the much-anticipated finale to "Lost." "Once it has officially aired, the narrative is out there for public consumption," he said, adding that viewers should not go to Wikipedia "if you don't want something spoiled."
The musician and mystery writer Rupert Holmes was less forgiving of Wikipedia's penchant for exposing his twists, whether in songs like "Escape (The Piña Colada Song)" or the mystery musical "Curtains."
"The rules of 'full disclosure' don't apply to fictional creations," he wrote in an e-mail. "If you give away the secret of a masterful magic trick, it is not as if you are protecting naïve consumers from wasting their money on a con artist. We want, even hope to be tricked, surprised, stunned. An illusionist is not selling us swamp land, miracle cures, junk bonds or Ponzi schemes. He is selling us the childlike thrill of believing, for one moment, that there really could be magic in the world."
He also questioned the motives of someone eager to report the surprise in a creative work, whether on a personal blog or a collaborative project like Wikipedia -- calling the achievement, at best, "a momentary sense of superiority."
"It's the self-aggrandizing vandalism of another person's potential pleasure. It's spray-painting your name across the face of the Mona Lisa and thinking you're one up on Da Vinci."
But trying to limit the effect of spoilers can be a huge task. A site like the Internet Movie Database, which began 20 years ago as a collaboratively created site like Wikipedia and is now owned by Amazon.com, is committed to protecting its audience from stumbling onto surprise endings.
It encourages the use of spoiler alert warnings on its summaries of movies, said Keith Simanton, IMDb's managing editor, and has a button that protects revealing passages from being seen unless someone moves a cursor over the text. But even full credit lists can inadvertently reveal twists in the plot.
"We get a lot of feedback from our users; that is one of the reasons we are so cautious," said Mr Simanton, who personally added a spoiler protection to the summary of the movie "The Usual Suspects."
David Thomson, the film critic who is the author of the reference work "The New Biographical Dictionary of Film," said he had learned not to give away the endings of the movies after hearing from "readers who call in or write in and say you are a brat."
Mr Thomson, who was born in Britain but lives in San Francisco, said he was less concerned about "The Mousetrap."
"I was raised to believe that everyone in Britain had already seen 'The Mousetrap,' " he said.
"I saw it when I was a child, and I can't remember the ending."