Click on a number which is next to (above, below, right, or left,
but not diagonal) the blank space. The numbered tile will move
to the blank slot. The object is to reverse the order of the tiles,
with 15, 14, 13, 12 on the top and 3, 2, 1, blank on the bottom
(and the other tiles in the same descending order).
To restart the game, click on RESTART.
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