Oh, Can You Do the Ken Ken?

I'm not sure when I first noticed the Ken Ken sitting there next to the crossword puzzle in the New York Times. At first I assumed it was a variation on the ubiquitous Sudoku, which I lost interest in several years ago after ODing on a particularly long plane ride. 

But then one day Josie (who had been doing them in math class, apparently) showed me how the Ken Ken works: like Sudoku, it's a square grid which you fill in with digits so that no number is repeated in any row or column. But unlike the Sudoku, there is arithmetic involved, along with pure logic. The numbers within each outlined box have to produce the target number shown in the box, using the arithmetic operation indicated. A 4x4 Ken Ken uses the numbers 1-4; a 7x7 uses the numbers 1-7, etc.

Try this one (ignore the yellow box; I can't get rid of it, somehow): 

See, that 12x box in the lower right-hand corner has to be 3 and 4, right? Which makes the two squares above it 1 and 2, leaving 3 in the left-most square of the 6+ box... And the 3+ box below that has to be 1 and 2, so the bottom square in that column has to be 4... And so on.

I quickly mastered the 4x4, then the 6x6 -- and I was hooked. Pretty soon Andrew was too, and after we had each had the experience of settling into a comfy chair with a cup of coffee and the Sunday 7x7, only to find the Sunday 7x7 already done, we started copying them out onto scratch paper so we could both do them. Our recycling is full of these hand-drawn grids, each scrawled over with our idiosyncratic chicken scratch notations.

Generally I do the 6x6 while I'm drinking my coffee in the morning -- a little logical ritual to get me ready for my irrational day, I guess. A month or so ago Simon, snuggling in bed with me, started asking questions. 

The Ken Ken is terrific multiplication practice, it turns out -- you're always having to figure out all the different ways three numbers could multiply together and get 24, or 60, or whatever. Simon, having spent the last two months memorizing his times tables, was intrigued by the idea that they could be put to use in this novel way.

It wasn't long before he was doing the 4x4 every morning himself (and I mean himself: woe betide the helpful parent who tries to give this boy a hint, or even ask a suggestive question, once he is bent to the task). Eventually he graduated to the bigger grids -- conscientiously copying them out onto scratch paper so as to avoid causing any comfy-chair-cup-of-coffee-let-downs. 

And then late Sunday night, long after he was supposed to be asleep, we heard a creaking floorboard out in the hall, and opened the door to discover our boy, sheepishly standing there holding this: 

Yes, instead of reading comic books under the covers with a flashlight, the child had furtively made his mother a Ken Ken. My geek heart swelled with pride as I marched him firmly back to bed and tucked him in.

Monday afternoon he came home from school with four more Ken Kens for me to do. He said he'd spent the day trying to figure out how to guarantee that each one had a unique solution. (This turns out to be a rather difficult problem -- he has mastered a couple of basic principles, but most of his puzzles still wind up with more than one end game. The Ken Ken publishers use a computer algorithm to guarantee that each grid can only be solved one way -- bonus geek points and a personalized puzzle to the first reader who can explain to us how this works!)

When Simon came home the next day with three more puzzles for me, I poured myself a cup of coffee and settled happily into a comfy chair. "When did you get so obsessed with the Ken Ken, Simon?"

"I'm not obsessed with the Ken Ken, Mom. You are."