Chain Reaction Grid

Chain Reaction Grid is a compact cascade puzzle. Every tile carries a number and an arrow. Tap one to fire it, and a pulse travels along that arrow until it meets another tile — which loses one, and fires in turn. At zero a tile shatters, and it still fires on its way out. Empty ground is skipped, so the board keeps re-routing as tiles vanish: the same arrow reaches further at the end of a board than it did at the start. Reading that ahead is the whole game.