Whether or not it is your turn, you may challenge another player who has just completed a move or set the Goal. The only two legal challenges are Now and Never.

1. By challenging Never a player claims that no correct Solution can be written regardless of how the cubes remaining in Resources may be played.
2. By challenging Now a player claims that a Solution can be written using the cubes on the mat and, if needed, one cube from Resources. A player may challenge Now only if there are at least two cubes in Resources.
   

   If a player challenges Now with only one or no cubes in Resources, the challenge is invalid and is set aside. The challenger is also penalized one point. Since a Solution must contain at least two cubes, it is also illegal to challenge Now after the Goal is set but before a cube has been played to Required or Permitted. In this case, the challenger is penalized one point and play continues.

A challenge block is placed equidistant from all players. To challenge, a player must pick up the block and say "Now" or "Never." Only the last player to move can be challenged. However, in a three-way match either of the other two players may challenge the Mover. To challenge a player picks up the challenge block (a coin, tile, or other three-dimensional object) and says either "Now" or "Never."

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