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The shell game (also known as Thimblerig, Three shells and a pea, the old army game) is portrayed as a gambling game, but in reality, when a wager for money is made, it is almost always a confidence trick used to perpetrate fraud. In confidence trick slang, this swindle is referred to as a short-con because it is quick and easy to pull off. 
In the shell game, three or more identical containers (which may be cups, shells, bottle caps or anything else) are placed face down on a surface. A small ball is placed beneath one of these containers so that it cannot be seen and they are then shuffled by the operator in plain view. One or more players are invited to bet on which container holds the ball - typically the operator offers to double the player's stake if they guess correctly. Where the game is played honestly, the operator can win if he shuffles the containers in a way which the player cannot follow.
In practice however, the shell game is notorious for its use by confidence tricksters who will typically rig the game using sleight of hand to move or hide the ball during play and replace it as required. Fraudulent shell games are also known for the use of psychological tricks to convince potential players of the legitimacy of the game - for example by using shills or by allowing a player to win a few times before beginning the scam.
As the game is almost always played dishonestly, it is typically regarded as a confidence trick and not as a legitimate game. There are, however, online examples of honest shell games due to the skill involved in visually tracking the movements of the containers.
The shell game dates back at least to Ancient Greece. It can be seen in several paintings of the European Middle Ages. A book published in England in 1670 (Hull Elections–Richard Perry and his fiddler wife) mentions the thimblerig game. In the 1790s, it was called "thimblerig" as it was originally played using sewing thimbles. Later, walnut shells were used, and today the use of bottle caps is very common.
The swindle became very popular throughout the nineteenth century, and games were often set up in or around traveling fairs. Fear of jail kept these shell men traveling from one town to the next, never staying in one place very long. One of the most infamous confidence men of the nineteenth century, Jefferson Randolph Smith, known as Soapy Smith, led organized gangs of shell men throughout the mid-western United States, and later in Alaska.
Today, the game is still being played for money in many major cities around the world, usually at locations with a high tourist concentration (for example: New York, Chicago, and Los Angeles, in the United States, La Rambla in Barcelona, Spain, Gran Via in Madrid, Paris in France, Kurfürstendamm in Berlin, Germany, Bahnhofsviertel in Frankfurt am Main). The swindle is classified as a confidence trick game, and illegal to play for money in most countries.
The game also inspired a pricing game on the game show The Price Is Right, in which contestants attempt to win a larger prize by pricing smaller prizes to earn attempts at finding a ball hidden under one of four shells. While the ball is not shown during the game, and the host shuffles the shells before the start of the game, contestants can win by either winning all four attempts or winning enough attempts, and picking the one that has the ball.
- Bishop, Glen, The Shellgame - For Tableside Tricksters, 2000
- Price, Paul, The Real Work: Essential Sleight Of Hand For Street Operators, 2001
- Whit Haydn and Chef Anton, Notes on Three-card Monte
- http://nypost.com/2014/12/26/three-card-monte-scam-artists-return-to-midtown/ New York Post, Three-card monte scam artists return to midtown, Is this Christmas 2014 — or 1974? New York City
- "Shell Game." Encyclopaedia Britannica. http://www.britannica.com/EBchecked/topic/539702/shell-game
- "The Rambla of the Thimbleriggers." Baquero, Camilo S. Translated from the Spanish by Summer Fingersmith. El País. http://www.robbedinbarcelona.com/2011/04/23/the-rambla-of-the-thimbleriggers/
- Trackbox for Android, Nontraditional shell game implementation for Android devices.
- Trackbox for iOS, Shell game for iPhone/iPad devices.
- Play the shell game, Play the several versions of the shell game and see video clips of the shell game being performed.
- How do big city shell games work?