The Origin of the KenKen Puzzle, a History of Building a Sudoku Competitor

Published Jan 2, 2024

Hi! Can you tell us who you are and about your game?

Hey everyone, hope you’re all doing well! I’m Robert Fuhrer, the founder of Nextoy LLC and part of the team that made and popularized KenKen—a classic puzzle game that you may have seen in the New York Times. I’m a toy and game conceptualizer and an entrepreneur, and you might have even played with some of the games I created, like Gator Golf and Crocodile Dentist!

KenKen is a puzzle game that works with basic math in mind. It has a grid filled with cages that have mathematical operations in them and have target values that you solve for. Its main objective is to complete the whole puzzle without repeating any numbers in any row and column, while at the same time hitting the target value that is within the cages.

Why did you create your game?

Back when I created KenKen, I was already an experienced member of the industry. At that point in time, I’d already been traveling to Japan, the birthplace of the predecessor of KenKen, dozens of times. And it’s safe to say that I’ve spent a lot of time there even before I discovered what would become KenKen.

I was in Japan at the time, and Tak Kubodera just got back from their trip to the Bologna Children’s Book Fair—the largest children’s book fair in the world. He comes up to me and talks a bit, and he tells me that they presented something to a bunch of companies in Bologna that garnered a lot of excitement.

He then showed me a book called the “Kashikoku Naru Puzzle”, and immediately, my eyes glazed over. I mean, I wasn’t a puzzle person. This was already around the time Sudoku was also rising meteorically in the U.S. market, but I still hadn’t played it. I knew about Sudoku even back then but I just wasn’t really a puzzle person. Just a look at the thing, with its hard lines, mathematical operations and all that… frankly it was an intimidating thing.

But then he says that the book sold 1.3 million copies in a ten-month period. He also pointed out its other features like its depth, scale, and how it was available and accessible for all ages and was gender-free… it was basically a thing that symbolized what I’ve been preaching about for years.

And in a not-so-brilliant moment for me, I said: “The book could be the greatest product in the world, but if no one knew about it, it wouldn’t matter, Tak-san.” He then asked how would I fix that problem, I told him I’d put it in the newspaper for all to see. Then Tak-san said: “Bob, please put it in the newspaper.”

I laughed internally a bit… then it dawned on me. Oh God, I was really going to do this.

How did you create KenKen?

So now that I had a goal, I set out to talk with Will Shortz, the New York Times editor. He lived near me, and he was already getting even more famous at that time for Crossword puzzles. Really, I was trying to stop this pitch about what would become KenKen, and the action of showing it to somebody in the New York Times was more of a throwaway line. But someone working for Kubodera, her name was Nikki, told me that she already sent it to the NY Times. And I told her that they wouldn’t get a response by sending it cold like that.

Afterward, I reached out to a friend of mine to introduce me to Will Shortz, and he just told me to e-mail him and gave me his email address. We communicated a little bit, and then I went to him in person to show him the puzzle and the process of solving it, but he then told me that I should just leave it because if he had to have someone show him how to solve it for him first, then it probably wasn’t anything worth it.

So I left, and three days later, I got an email from him telling me that once he had it in his hands, he couldn’t put it down! He even told me that out of the 105 puzzles in there, he’s already solved the first 103! I came back to him, told me that it was an amazing puzzle, and then he set me up with a meeting with the NY Times.

As to why it got called KenKen, one of the Japanese editors called it “Ken Ken Blocks”, and in Japanese, Ken means “wisdom”. So Ken Ken meant “wisdom squared”, and intuitively, I ended up calling it KenKen.

Difficulties Behind the Emergence of KenKen

Unfortunately, the NY Times didn’t add the puzzle to their newspaper. Will also added that even if they added it to their publications, it wouldn’t be in the newspaper as it would be too much of an undertaking, so his best bet was that it could be published in the Sunday magazine, and the very last page.

I was dejected over it, the math wasn’t working, and there were too many factors to consider, not to mention I also had to find a way to involve the Japanese creator, who is Tetsuya Miyamoto-sensei. But I knew I had it. If Will Shortz himself guaranteed that it was a good puzzle, then it’s definitely worth pursuing. So… I slept on it.

The whole project went on ice for a time, until I met David Levy from the United Kingdom, the father of computer Chess. He was very skilled in tech and artificial intelligence. I reached out to him with the encouragement of a friend, and a lot of things happened involving a writer’s strike, the book he wrote, the TV show Colbert Report, and even Sudoku. But even through a lot of obstacles, I got another solid thing—even the father of computer Chess loved KenKen! So he told me that the Times UK was looking for a successor to Sudoku, and he pitched it to them… with no success as well.

But it wasn’t all negative! After a while, I got a phone call from McMillan Publishing in New York, and the woman on the phone told me that Will Shortz said that I had something they must have. So it went down, we made a 10-book deal with them… it didn’t do well, but it was still something.

There was also the issue of the rights to KenKen. Trying to publish it meant it had to be trademarked to someone or a company. Then Tak Kubodera called me to talk about progress, and I told him we’re generating interest but there wasn’t a concrete outlook for it, and even if it did get traction, it would need a significant investment of at least $200,000. Then came his words: “Can you guarantee success in this?”, because his company couldn’t put down a sizable investment based on speculation.

I… I don’t know. I felt something in me, there was an opportunity. It was there, in my bones… a nagging feeling. So I said that I’d do it. And since I’d be the one doing it, KenKen would be registered under my name. I’ll pay for it all and work out something. And Tak Kubodera gave his blessing.

We figured out an arrangement, which we still have to this day, and it did take over a year to get their contract done because there were a lot of people weighing in on it.

The Gamble Pays Off

After all that, I called up my lawyer to trademark KenKen under my name, and we discussed a lot about the legalities and whatnot… it was a very complicated process, and we took three categories for it; computer, print and play, and order. Of course, we started in the U.S. but we started registering everywhere, like South America and other countries. All that meant a lot of fees. Fees for renewal, maintenance fees and more.

Throughout all of that, I thought I made a foolish expenditure. We didn’t have businesses in a lot of these places we registered in, and thinking about this thing that we weren’t seeing returns from, it started to make them. KenKen was gaining traction, at long last!

Our team also had become well-rounded, we had David as the tech expert, with me, Will, Mr. Miyamoto, and the others handling everything else like operations, publishing and whatnot.

We launched in the Times UK in 2008, and also in the U.S. in the same year. I also decided to build, but the website had no game in it—it was all just information and the rules for playing the puzzle. So we now knew that KenKen would primarily be in digital form, as making physical copies would be too much.

With that, David oversaw the development of the tech side along with the specifications of Mr. Miyamoto, and soon, we had the Kenerator—a complex artificial intelligence computer program that generates and checks every new puzzle.

After that, we went back to the New York Times again to pitch KenKen into their publications. It was still a no-go for them, so I told them: “Why not put it up on your website because it won’t take up any real estate? It’s just a game.” After a bit of deliberation, they caved in and said they would give it a try.

Luckily, I also gave them an ultimatum: within 60 days, you either take it and get exclusive printing rights for a year with some restrictions since the Times UK already launched it before them, or you leave it forever. And like a month in after I told them that, they said that they wanted the print rights!

We got that smoothed out and they were scheduled to launch at the beginning of January 2009, then it got delayed. And again. And again. A tad frustrated, I talked to them again, and a woman named Lydia Reynolds received me. I asked if they were seriously going to go through with it, and she said: “We are. Seriously. Bob, we have more people trying to put KenKen in the New York Times than we have covering the war in Afghanistan!” And they finally launched on February 9, 2009.

From then on, KenKen just continued on an upward trend. From the little website that had 250 visitors on a good day to 35,000 and getting easily a thousand emails. That’s that. The long, arduous journey of KenKen has now found its stable path.

How did the revenue stream of KenKen work?

That was a doozy at first. I actually brought in a guy who I grew up with who was in business and studied at the Harvard Business School, and his name is Jerry March. Jerry basically straightened out the mess I made!

So our primary income is through digital advertising, but we also have other revenue streams through self-publishing and license publishing. Though we don’t do much of the latter two anymore, but it’s still available for the public. So KenKen as a business really is held up by digital ads.

Were there any significant prior games or involvements you had before KenKen?

Oh, there were a lot! I didn’t create KenKen and tried to popularize it right off the bat, I worked with many different people and gained tons of experience before I even encountered KenKen. Milton Bradley and the others… I’ve been around in the toys and games industry and I’ve made lots of good splashes there, like just I said before, Gator Golf and Crocodile Dentist.

There were a lot of things that went on right before I created KenKen, but the time I spent working with the licensing agency handling Othello and meeting the Japanese… that’s when the path of my career started becoming more progressive.

What was your experience with Othello?

My father knew the agent for Othello back then, and I was hired as a product manager under them. I worked for the small company that they were at the time, and those four years of experience under them were crucial as Othello originated from Japan. The Japanese would send their corporate representative to learn about the U.S. market for their business, and when that rep was here, my job was basically to befriend them.

And that relationship would be very pivotal for both me and the company I worked for, as we ended up working very closely with one of the biggest Japanese companies at that period, and we would do so for 27 years. That specific part of my career was very significant and was key to jump-starting my career in toys and games.

You worked alongside the Japanese for a long time, how did that start?

My dad was working as a liaison for a business in Boston around 1981, and a Japanese company reached out to him. There was this person named Kono who lived in the U.S., and he was called by a Japanese man named Tak Shimakata to tell him that he had just been assigned to run the international division of that Japanese company. Shimakata told Kono that he wanted to be introduced to people in the U.S. market, and he handed my dad’s contact information to him.

Then Shimakata’s secretary scheduled a dinner meeting, and my dad asked me if I could come along, I didn’t have much going on at the time so I went ahead and joined them. During the dinner meeting, my dad and Shimakata were not hitting the same wavelength business-wise—my dad worked with toy rockets at the time and Shimakata was presenting vehicles that were a completely different category from the field my dad was working in. Seeing that, I hyped up and provided insight on which products should go to where like this thing should go to Tonka or that should go to Remco Industries, and stuff like that. And that’s how me and Tak Shimakata started working together.

He was leaving for Europe at that time, and he asked me whether I could get the production samples he brought along to the companies I mentioned. Sensing the opportunity, I accepted and took the pieces and sent them to a bunch of different toy companies, and I ended up getting a bite from a company named ERTL, they’re no longer in existence now though.

So ERTL’s representative, Jack Stoneman if I remember correctly, told me that they were working on a piece just like this, but Shimakata’s samples were already working and complete, plus they didn’t need batteries. Long story short, I fumbled my way through the business deal, I contacted Shimakata, ERTL and Shimakata secured a relationship, and my first-ever attempt as a middleman of sorts and facilitating a business connection ended up being a success!

Japanese culture and building relationships

As one of the rewards of that success, they provided me with a trip to Japan in December of 1982; the first of what would be a hundred and more so that I would make towards there. When I got there, I was given another project to handle, then another, then another… we got down the road with one of the biggest toy companies there. Slowly and surely, things grew big and I got formally introduced to the company. I would work with this known group for 27 years and learn so much about the Japanese, their culture, and of course, their market.

Unfortunately, around 1996, Mr. Shimakata died from a brain aneurysm. But the good thing was that one of the senior staff reporting to him took on a leadership role, and I continued to work with him more than 40 years later when we first met. That whole period in my life is such a wonderful memory. So I took like, 140 trips in 40 years, it was like five trips a year, and I continued to do so since 1982 until COVID struck. I haven’t been back to Japan after the pandemic.

So within those 40 years and more of working with the Japanese, they pretty much gave me free reign of their Overseas Operation Toy Division, OOTD. I had very little to do with the company’s operations in Japan itself but in the OOTD, I had lots of talented people working with me and for me. Thousands of ideas and pitches went through our team, and we all had to whittle them down to reveal the very best among them in terms of possible public reception.

And I tell you, it was hard work because nobody comes up with ideas like the Japanese do. They have really crazy concepts there, and even their commercials were very unique. For me, one of the most memorable pitches I handled was “My Dog Can Talk”. It was basically a collar with a speaker! You’d talk to the other end of the leash, and the collar’s attached to your dog, right? And it would sound like your dog was actually talking. Hilarious stuff!

On that note, I still have these old archives that I go through once in a while. But the majority of them, I donate to the toy museum in Rochester, NY. It’s called the Strong National Museum of Play.

What were the main differences between Western games and toys from Japanese ones?

There’s a thing we like to call “Watch Me” toys—like, you wind the toy up with this wonderful mechanism inside it and it would hop three times or do a flip or something like that. The Japanese toy industry was filled with it, and their market marveled at the possible intricacies involved in the mechanisms of the toys.

However, Western toys were not like that. In fact, at the time, U.S. toy manufacturers weren’t interested in that at all. So what were we to do? We didn’t want to force the Japanese standard onto the U.S. market, so instead, we played with those mechanisms and applied them to games or toys that would appeal to the Western audience instead!

Based on your experience, are there any pointers you can give about working in a foreign market like Japan?

There are so many things you need to learn about their culture beforehand to even start effectively working with them. See, here’s the thing. They have a saying there that directly translates to “the nail that sticks out must be hammered down”. If you’re a Westerner like me, we’re brought up with a culture of individualism and self-merit, but in Japan, everything must be made with a team-oriented mindset.

Standing out, especially in a work environment, can actually be detrimental to you when you work with the Japanese. Unlike in the Western world, where you want to stand out and gain recognition through individual efforts and traits, with the Japanese you have to become one with the team and move in one motion and one organized direction. To them, when you’re a part of a team, every decision taken is a team decision, and individual decisions are not common.

Another thing is that you also have to know about their subtle social cues. We Americans, for example, are raised with the idea of sincerity to be a straight look in the eye and a very strong handshake. We shake firm and maintain eye contact not to intimidate, but to show that we are resolute, offer respect and want to be respected in return. Doing that in Japan can be taken in a very negative way, due to the vast cultural differences. Seniority weighs a lot more in Japan, and showing respect can be done in different ways, especially with team activities like drinking together after work.

And trust me, it was challenging to learn all of that, but it was worth it all to learn and be molded alongside my Japanese colleagues.

You founded Nextoy—how did that come to be?

Even when I was young, I already had an entrepreneurial spirit. I had little businesses and whatnot during the summer to make some cash and stuff like that. To be frank, I feel that I’m not at my best working for somebody. Yes, I did work as an employee for four years, and I was good at my job. I was a model employee. But I wasn’t driven by it—I couldn’t see myself being on a company’s payroll as a worker for the majority of my career.

I never liked being told what to do. Heck, I don’t think I was even a good student when I was a kid. I was independent at heart and wanted to control my own destiny, regardless of the uncertainties that came along with being free. And Nextoy was my way of taking control of my life.

Most people, when asked about their motivation behind establishing their businesses would say: “I wanted to make money”, which I think is not the right motivation. For me and the entrepreneurs that I know, embarking on a business venture is not about profit, but the independence it provides.

Don’t get me wrong! Of course, profits and the financial stuff are crucial! After all, the business can’t uphold itself without money. However, in my opinion, profit shouldn’t be the sole driving force behind a company. Money is just the results given to you and your business when you’re doing something and doing it well. Aside from that, when it comes to profit, I also think that luck and relationships play big roles too.

And in this regard, I had both luck and great relationships to work with. I had an idea for a game that was going to need some time to develop with a friend, and we did. It came out several years later, and it was a type of memory game we called “Locomotion”. A company named Schaper, now defunct, sold it for a while, and we had high hopes that it would become popular.

Sadly, it’s quite normal to create something in this industry that you think will end up big but… well… it doesn’t become so. But that didn’t stop me from continuing, as the process was fun and worth it for me.

Could you tell us more about your philosophies?

Sure thing! As I said, my primary driving force in life is to be free. It’s my philosophy. And I’ve even taught that same philosophy of freedom to my three kids, and they’ve all grown to have great work ethics. I’ll tell you all now what I’ve also told them before: freedom is also a currency. When you’re young, you can afford to do a lot of things with the freedom you have. As you age, that currency of freedom has already been traded for something else, like experience, relationships, more opportunities and other stuff. 

For me, I ended up making a lot of good relationships, and I proudly raised my children. Like real-world currency, your freedom currency also has its highs and lows. When you get to my age, where the children you raised go out into the world on their own, that currency returns significantly and you have more leeway to use it again.

So whether you’re old or young, use your freedom as you prefer and make the most of it while you can.

In your opinion, what makes a great game?

Oh, I’m not sure if I’m the best person for that question, but of course, I’ll share my insight on it. When it comes to games in the toy industry, at least from my perspective, we don’t go straight for the gameplay necessarily, but more on what marketing hook or feature it has that can sell. In my field, there are a lot of times that certain games and toys are great to play and have good depth to them. However, they may not be the kind of thing that you can pitch and sell to a manufacturer.

In my industry, back then, toy and game inventors had like some sort of special circle or fraternity, and all the big toy names like Hasbro, Mattel, Spin Master, Goliath… they would be buying or getting ideas within that group. It didn’t matter if your game or toy had great playability and depth, the question was whether it was mass market-friendly and promotable.

The Success of the Crocodile Dentist

A great example of this is one of our more famous products, “Crocodile Dentist”, which was designed by an excellent industrial designer. His name is Phil Grant, and he used to be in Milton Bradley, and he’s a very critical designer. At this point in time, we could have changed to different, more mass-friendly designs like a dog or a dinosaur, but no, he stuck to a crocodile. He made it in such a way that it became a commercial success that still sells even today. 30-something years later, and it’s still selling! Now that’s a great game.

So from that success, Milton Bradley wanted to ride off of that success and created other products striving for that same level. So what came next was “Shark Attack”—it was a board game where there was a shark and it was chasing the characters and you had to roll the dice and move. It was another huge hit, so they tried to cash in with the Shark Attack brand, and made “Shark Attack Bowling”.

Now here comes the interesting part. Shark Attack was a success, right? So Shark Attack Bowling, a well-designed, entertaining toy, at least in my opinion, riding off of the high made by its predecessor was surely going to turn out good, right? But, sadly, no. It didn’t do well. It actually only lasted three months then they pulled the plug on it. And based on that commercial flop, other sports-related games and toys were now also in the line of sight for getting shut down.

And at that time, the same designer who made Crocodile Dentist was creating another sports-related game. The higher-ups already had that unspoken rule in their minds that sports games riding off of a known brand are sure to fail. But no, Phil didn’t stop. He kept at it and continued pitching in the meeting, and he ended up designing another classic hit—Gator Golf. They just changed the branding from a croc to an alligator, and it became another well-known hit among toys.

So yeah, based on my experience, a good game or toy is something that captures the heart of a lot of people in a fast and lasting manner.

What games do you like to play?

I’ve always liked playing games centered around deductive logic, like Stratego and Battleships. Ah, right! Guess Who! The one with the cards? I really like that as well. Though for modern games, I can’t see it the way young people do, as I grew up and made a career looking at them from a commercial standpoint.

Is there any advice you can give to people who want to be like you?

Oh definitely. I’m a dreamer. Always have been. Aside from the philosophies that I imparted earlier, here are two things I’d like to share:

1. Persistence

Persistence really is the name of the game. However, you need to know the delicate balance between persistence and nuisance, and that is something you learn with enough experience and patience. But know this, as long as you believe in the potential of something, that nagging feeling, try to see it out to the end, even if you’re lacking in the confidence to even attempt it.

2. Don’t be afraid to collaborate with people

Collaborating with people and making connections is what made KenKen what it is today. What made me into what I am today. I understand the fear behind trusting someone with something important and special, and really, there are so many horrible stories about trusting the wrong people. But still, don’t be afraid to try and work with people, and all the while, gauge the people you’re attempting to build something with.

Where can we go to learn more about you and KenKen?

Of course, if you want to play the classic KenKen puzzles, go to! It’s a complete website with information on the game and on us, the KenKen Team. And if you have any questions, game-wise, technical-wise or commercial-wise, you can go to to learn more.

Have a game to sell?

Let’s find out if we play well together.