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Liquid is open-source and licensed under the GNU GPL version 3. The game is played by using the bucket tool to fill territories with your colour of water. Once a territory is full, filling it with more water causes the weakest wall of the territory to collapse. The adjacent territory whose wall has been collapsed is absorbed into the exploding territory, and its water is recoloured to your colour. You win when the entire map is filled with your colour. If you have no legal move, then you lose.
Walls are coloured by strength—violet is strongest and red is weakest. Hovering over a territory will highlight the weakest wall.