- Place one orb per turn in any empty cell or a cell you already own.
- Each cell has a critical mass equal to its number of cardinal neighbours — corners 2, edges 3, centre 4.
- When a cell reaches critical mass it explodes, sending one orb in each cardinal direction and converting those cells to your colour.
- Explosions can trigger chain reactions — keep going until the board stabilises.
- You win when all opponents have no orbs left on the board.