Stack'em 3D is a game app in which you need to score four spheres in a row - vertically, horizontally or diagonally in three dimensions. In a score four game in progress, one aims at forcing a win by making two threats simultaneously; conversely, one should prevent the opponent from doing so. As a general rule, discs played in the center columns are more valuable than border columns, because they participate in more potential four-ball lines (and accordingly limit the opponent's score four opportunities).
Among good players trying to score four, the short term goal is to connect three balls, thereby preventing the opponent from playing in a certain column. Towards the end, the game then often turns into a complex counting match: both players try to score four by forcing the other to play a certain column. In these situations, it is useful to realize that, if it's your move, then after filling an even number of places, it's still your move. Every column has an even number of places.
