The Apple App Store has just released designer Stan Resnicoff's latest game: TACTX. I was a beta tester on this clever twist on Go, Pente, and other place-a-stone games. This one is played on grids with different patterns of black squares (like crossword puzzles, but there are no letters or words). Take turns placing marbles on the white spaces. Earn one points for each marble when you create overlapping rows, columns, and diagonals. The goal is to bank the most points by the time the grid is completely filled in. The most challenging level is the final, completely blank game board. Sure, you can easily beat the computer at the start, but look out–the unrelentingly clever algorithm will likely turn the tables on you by the end. Like the best games TACTX is easy to play but hard to win—and addictive. The sweet, hard-fought wins are the perfect intermittent positive reinforcement that keep you playing.