This game started as a pencil and paper game, but it's wonderful for computers because the challenging levels of the game require a good deal of erasing. I've been playing GameHouse's version, here. It's the nicest interface; unfortunately the highest level is no longer particularly difficult for me, so I guess I'll have to go hunting again.
I suppose that everyone knows that Suduku requires that you put numbers on a grid such that each row, column, and square have the digits 1-9 in them with no repeats. With a variety of kinds of deductive logic, you can complete almost every grid without guessing.
As long as you don't make a single mistake. Suduku is one strike and you're out. Put the wrong number in a box and continue the puzzle and you'll never figure it out. Nor is it usually possible to figure out where that mistake was without starting over. Suduku teaches a person to be careful. The pencil and paper game teaches a person to be neat with tiny little trial numbers. My son taught me to use one of the nifty new mechanical pencils now on the market; they stay very sharp and come with the most amazing erasers. My, how times do change.
Being a Universalist, I object to "one strike and you're out." The thing that has most puzzled me over the years about Christian orthodoxy is how Christians can talk about God, the loving father in one sentence and "If you don't believe (not to mention, if you commit any number of sins) you're doomed to everlasting punishment." Everlasting! Everlasting is longer than any loving parent I know would punish their child.
But I do like Suduku, which is, after all, just a game.