The Shell Game is a City Game much like Chess or Poker, in which Aiden has to follow a ball hiding under one of three cups. Successfully following the ball will lead to extra cash for Aiden and unlock certain skills. There are 5 levels in total, with each having three rounds. Successfully completing all three levels will increase Aiden's XP.
The way the game's graphics engine renders items offer an easy way to win at the shell game. Simply turn up the graphics settings in the options menu all the way to the maximum, then play a shell game and pause after the ball has been moved and you are left to select a cup. Upon pausing the game you will see that the ball has been rendered underneath one of the cups. You can also turn the brightness up on your TV/monitor to see it easier. Just pick this cup to reveal the ball underneath. This works on all difficulty levels of the shell game.