More than six players could play, but it becomes difficult for all players to see the cubes. Two to four players is probably best. Special 6-sided dice with numbers and arithmetic symbols on them are rolled out at the start of each round. A 'goal' expression is formed, and then players take turns moving one cube at a time to one of three areas: Forbidden, Permitted, or Required. Each move is supposed to keep possible the ability to eventually form an expression equal to the goal, but it must not be possible to form such an expression immediately if there are still cubes remaining in the resources pile. Players may challenge each other if they believe a move has rendered impossible any expression that equals the goal, or if an expression can be formed prematurely. When a challenge is issued, all other players decide which side they want to be on. The side claiming that an expression is possible, has the burden of proof to create such an expression. The final expression must use all the Required cubes, none of the Forbidden cubes, and as many Permitted cubes as desired. Expressions are written down the cubes are not moved. Parentheses may be inserted as desired. Rounds continue until some agreed condition is met (high score reached, or number of rounds). - BoardGameGeek