Neelix moving the cups while he demonstrates the shell game
The shell game presents itself as a gambling game, but in reality, it is a scam. The victim of the scam is often referred to as the "mark".
Items required for the game include three small opaque containers, a token, and a flat surface. The game begins with the perpetrator, often called a con-artist, placing the token under one of the three containers, then quickly sliding the containers around until the container that holds the token is uncertain. The object of the game from the mark's perspective is to guess which container holds the token.
The secret of the game is that none of the containers hold the token. The con-artist has removed the token through sleight of hand. No matter what container the mark picks, the token will not be there, and he will lose whatever he has wagered.
Often, an accomplice to the con-artist will play the game to build the mark's confidence in the possibility that the game is winnable. In reality, the con-artist makes sure that the token is under the pre-determined container when the accomplice is playing.
Neelix and Tom Paris unsuccessfully used this scam on The Doctor, in an attempt to prove their deceptive instincts had not dulled with time, after they were conned by Dala and Mobar. (VOY: "Live Fast and Prosper")