Turnablock is a very simple game for two players invented by John Conway. It is played on a 3 by 3 square board with 9 counters that are black on one side and white on the other.

The counters are placed on the board at random, one per square, except that the majority must be black uppermost.

Players take turns to reverse all the pieces in a block (e.g. $1 \times 1$, $2 \times 3$ or even $3 \times 3$). The object is to have all pieces white uppermost on completion of your move.