Either complete Venetian canals or else determine the task impossible. 

Venice Connection is a river-building / tile-placement game for two players. Your goal is to complete the canal in a loop. The game consists of 16 identical tiles, each with a straight section of canal on one side and a bend on the other. All the tiles are available for either player to use, and on a turn you place 1-3 tiles connected in a straight line adjacent to at least one other tile already in play (except on the first turn when you just place these tiles on the table).

If during the course of the game, one player notices that the canal is now impossible to complete, they say "impossible" to the challenged player (who placed on the previous turn), who now must try to finish the canal alone. If the canal can be completed, the challenged player has won; if not, the challenger has won!