As with many other challenging feats, there are those who have to push the boundaries of Sudoku. If you're just starting out, you might want to refer to my earlier Sudoku introduction instead.

For those who are curious about where else Sudoku can go, read on!

The first obvious variation would be to use letters instead of numbers. This would allow a square of 25 by 25 to be built! If you've tackled standard Sudoku puzzles before, you can see whether you're ready for an alphabetical Sudoku puzzle here.

Since magic squares are brought to mind, here's a PDF file of a Sudoku puzzle with the added constraint that 1-9 can only appear once each in each of the main diagonals!

Is it perhaps the 2-dimensional nature of Sudoku that no longer challenges you? If so, try the Dion Cube (PDF), in which you solve not only the faces, but the insides of the puzzle, as well! Don't worry, though, as there is a solution available, but no peeking before you try it, OK?

If those don't pose a challenge for you, then I suggest reading Sudoku Variations over at MAA Online, which has enough variations to keep you busy for a long time. A personal favorite from this page is the Domino Sudoku puzzle which is pictured above.

Even magicians are realizing that Sudoku opens up a world of new presentations. Just as they did with Rubik's Cube, magicians are finding ways to capitalize on Sudoku's popularity. Magician Justin M. Monehen has a new routine available called Sudoku: The Ultimate Mental Workout. In this routine, a random Sudoku puzzle is chosen from a puzzle book, and the magician studies the puzzle for a few seconds, and is then able to solve it in seconds! Several variations are included, including one in which an audience member intentionally miswrites one of the numbers, and the magician is able to point out which one it was.

I think part of what fascinates me about these Sudoku variations is the creativity involved in coming up with them, as well as working the solutions out to make sure they're even possible. Until I hear of a Knight's Tour on a Sudoku board, that's all for now!