Solving this puzzle requires the use of Conway’s Game of Life for five rounds, until the board reaches a stable conformation. Conway’s Game of Life is a deterministic game with three rules (this wording taken from Wikipedia):
- Any live cell with two or three live neighbours survives.
- Any dead cell with three live neighbours becomes a live cell.
- All other live cells die in the next generation. Similarly, all other dead cells stay dead.
After five iterations, the remaining twelve cells are in a stable configuration and will not change further. Reading their letters from top to bottom, left to right give the answer FLAMETHROWER.