Three light sources of different colours stand at the edge of the board. Each move you place one mirror on an empty square or put it in place of a mirror facing the other way, and the mirror turns the beam at a right angle. When a beam reaches a crystal of its own colour, the crystal lights up and disappears together with every mirror on that beam's path. A crystal of another colour blocks the beam. Before you place a mirror, the game shows under the mouse or your finger where the beams will go and what will light up.
After every move new crystals appear, and the longer you play, the more of them arrive at once. The game ends when the new crystals no longer fit on the board. Lighting several crystals in one move scores more than lighting the same crystals one by one, and no new crystals arrive after that move.