Reversi

Reversi is played on an eight by eight grid cut into a slab of slate, set flush into raked white gravel. Each stone is a river pebble — matte black basalt on one side of the table, pale quartz on the other — and every one of them can change hands. Place a stone so that it traps a straight run of your opponent's between it and another of yours, and the whole run turns over. Lines go in all eight directions, and a single stone can turn several of them at once.

The trick is that turning the most stones is usually the worst thing you can do. Every stone you flip is a stone that can be flipped straight back, and a fat pile in the middle of the board leaves your opponent squares to play into. The players who win count squares instead: they take the edges, they leave the four squares beside each corner well alone, and they wait until the other side has nowhere good left to go. A corner can never be turned, which is why the whole game bends towards them. With no legal move you simply pass, and when neither side can move the stones are counted and the larger colour takes it.

Play Black or White against Easy, Normal or Sharp, or pass the device back and forth with a friend — in face-to-face mode each player's counter turns to face them. Legal squares show as faint dots you can switch off, the stones that would turn are outlined before you commit, moves are written down in a1 to h8 notation beside the board, and a bar along the bottom shows the count swinging back and forth. Twenty-four puzzles set you a position with one best stone, from quiet openings to endgames you can read to the last square. The Sharp opponent searches several moves deep and solves the last twelve squares exactly, so the finish is never sloppy.