- 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 surrounding squares — corners 3, edges 5, centre 8.
- When a cell reaches critical mass it explodes, sending one orb to each surrounding cell (including diagonals) and converting them to your colour.
- Explosions can trigger chain reactions — keep going until the board stabilises.
- You win when your opponent has no orbs left on the board.