You play on a 6x6 board. You and the computer (or another player) take it in turns to tap a square to place one unit of radioactive mass (e.g. plutonium) on it. When a square reaches Critical Mass it explodes. The fallout lands on the four squares above, below and on either side and they will also explode in a chain reaction if they have reached Critical Mass. If fallout lands on a square owned by the opposition, then you capture the square. The game ends when a move results in all the opposition's squares being captured. Palm OS 3.5 or higher. Color or mono. Trial version limits game length to 20 moves.