Fifteen - a kind of mosaic puzzle. As in any other puzzles, this is necessary to collect image of the individual pieces. Pyatnashek feature that all fragments are rigidly attached to each other, and to move and need to use a blank cell. Moving the fragment to an empty space, you loose another cell that already can stand next fragment. Fifteen - is a tricky puzzle, the farther you advance, the harder it is to collect it, and the last piece is the most difficult.
