Imagine a three dimensional version of noughts and crosses
where two players take it in turn to place
different coloured marbles into a box.
The box is made from 27 transparent unit cubes arranged in a
3-by-3-by-3 array.
The object of the game is to complete as many winning lines of
three marbles as possible.
How many different ways can you make a winning line?
A poster of this problem is available
here.