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3D Tic-Tac-Toe is a version of tic-tac-toe that has been extended into three dimensions, but is otherwise identical to it's two dimensional counterpart. The easiest way to visualise this is a cube divided up into 27 smaller cubes, which are arranged in a 3x3x3 array. Players take it in turns to place an X or an O into a cube, and the first one to place three in a row wins.

The above write-up, and the one in "How to always win at 3D tic-tac-toe" are true only for a 3x3x3 grid. Nontrivial (and fun!) 3D tic-tac-toe should be played on a 4x4x4 grid. Of course, then the objective is to get four in a row. It is simple to draw the board on a piece of paper, but takes a bit of time to learn to see the board in three dimensions. In high school I spent many hours of lunch periods (and history) playing this.

Below I sketch the board with one of the more difficult to see winning positions. (This wasn't a real game, which would have gone on longer.) You should think of the first board as the top board, and the last as the bottom. See if you can find the winning four in a row. There is a hint at the bottom.

```-----------------
| X |   |   |   |
-----------------
|   |   |   |   |
-----------------
|   |   | O |   |
-----------------
|   |   |   | X |
-----------------

-----------------
| O |   |   |   |
-----------------
|   | O |   |   |
-----------------
|   |   | X |   |
-----------------
|   |   |   |   |
-----------------

-----------------
|   |   |   |   |
-----------------
|   | X |   |   |
-----------------
|   |   |   |   |
-----------------
|   |   |   |   |
-----------------

-----------------
| X |   |   |   |
-----------------
|   | O |   |   |
-----------------
|   |   |   |   |
-----------------
|   |   |   |   |
-----------------
```

Hint: X has won.

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