My brother lives in Minneapolis, and we visited him last January and this. Both times we used the light-rail train to get to the airport. Last year he had to drop us off pretty early, so we stopped at the Mall of America. We'd arrived too early and most of the stores were still closed, but we wandered around anyway. We found a toy/game store called

Marbles - The Brain Store, and I knocked on their doors to ask if I could just come in and browse. They had a good collection of my favorite sorts of toys and games, and they ended up letting me buy a few things,

Spot it among them.

It's not a math game at all. It's a contest to see who can find a matching picture first. My son is better at it than I am, so he enjoys winning. Every time I played, I pondered the math question that stared me in the face:

**How did they make sure every possible pair of cards would have just one match?!** But I didn't know how to get started, so I never really pursued the question until recently.

Spot it is a good travel game, so I packed it this year as we headed off to visit family and friends in 4 Michigan cities, a Chicago suburb, and Minneapolis. My old friends Chris and Paul have twin girls my son's age, and our visits to their backwoods home are always a rich experience of outdoor play, good healthy food, and deep conversation. Chris and I decided one evening to look at how Spot it is put together. Chris doesn't think of herself as a mathematician, but she organized the information in a way that helped me solve my problem. It was also a great motivator to think through it together. We used numbers to represent the different pictures, and drew our cards in different layouts, looking for patterns.

We worked on it for a few hours, and got some good insights, but didn't solve it. That was enough to get me hooked. I got my mom to work on it with me. We counted all the pictures, and found 57 different pictures. Then I worked on it on the train to Chicago, and got some good stuff figured out.

One thing I did was to make a mini-version of the game. My 'cards' (circles with numbers in them), used only 4 numbers each. I figured out that I had to use 13 different pictures, and could have a deck of 13 cards - all different. That gave me enough push to find a scheme for the bigger deck. But I wasn't at all sure it was right. There are an awful lot of possible pairs when you have 55 cards. (

**How many?**) How could I be sure I had exactly one match for every pair of cards? I

*thought* I did, but I sure didn't know how to prove it.

That's where I was with the problem when I visited

Prairie Creek Community School, about 40 minutes south of Minneapolis. Michelle Martin (who has a chapter in

*Playing With Math*) let me show the kids (4th and 5th graders) the problem, and they all dug in. They pulled out all the cards with a heart - there were 8 - and thought about what they now knew. 8 pictures per card, one of which was the heart, along with 7 others, all different... They decided there had to be at least 57 different pictures (7 unique pictures/card*8 cards + the heart). I liked that they figured that out a better way than I had. The kids worked on the problem so diligently, but had to leave it and go on to other things. (And it didn't occur to me to leave my game there, so they couldn't keep working on it after I left.)

I went back to trying to find ways to prove I had a valid solution. My new burning question was 'Why did Spot it only have 55 cards, when it could have had 57 cards?' I put my 'cards' on Excel. But I still didn't know how to test each card against each other card. I understand that the macros in Excel use visual Basic, and I hoped I could do something with that, but I was once again pretty stuck.

I asked a colleague to help me get started on the programming, and he recommended Python. I couldn't get started on my own. I found the Python documentation very confusing, and got stuck every step of the way. Yesterday we had a bit over an hour between meetings, and he showed me how to get started with Python. He's learning it too (and he's a computer science teacher). So the best thing he showed me (probably pretty obvious) was to google 'python example x' every time I had a question. This morning I got my program working, and it verified that

**my method worked**!!! Maybe next year I'll generalize this to cards with n pictures on them.

The biggest lesson for me in this adventure was that working with others is a huge motivation for me. But I knew that.

[You may have noticed that I haven't given many details of my solution. I'm hoping you'll all play with it yourselves, of course. If you'd like to discuss all the gory details, please email me at mathanthologyeditor on gmail with your solution ideas.]