Cosmic Buddha pointed out this freaky puzzle/game on a web site. You are asked to pick a two-digit number. You then add the two digits of the number together (for example, if your number was 87, then you add 8+7), and then you subtract that sum from the original number. Then they show you a chart where each number is matched to a symbol. You find the number with your symbol, wait a moment… and then the web site magically displays an enlarged version of the symbol you are looking at!

Seriously, it’s kind of freaky at first. You start to wonder how they know what number/symbol you’re looking at. At first I thought it might have something to do with where your mouse is–that most people might hover their mouse over the symbol. But that wasn’t it. The game worked every time I tried it honestly. If I cheated and chose a symbol at random, it didn’t work.

Go ahead and check it out. If you can’t figure out how they manage to do it, then click through to see below the fold and I’ll explain…

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Okay, here’s how it works. As it turns out, adding the digits from any two-digit number and then subtracting that sum from the original number always gives you a number divisible by nine as the result.

So when they display the chart, they always have multiples of nine display the exact same symbol, and that’s the symbol that they later highlight. If you did the required calculations, then your symbol will always be the one they highlight.

They add a nice twist to it: the chart changes every time. So if you do it again, you will probably end up with a different symbol every time. Because of the complexity of all the symbols, few people will notice (at least right away) that the chart is being changed around. So because you get different symbols each time, you get the illusion that you really are getting a different result each time, and the game is correctly guessing your symbol each time.

You also probably won’t catch on to the multiples-of-nine gag because by the time you choose a new number, add and subtract in your head, and get a new result, you’ll have forgotten what the last number was. If your memory is better than mine, then you might notice it sooner.

I started catching on when I asked Sachi to come over and we did it together. Each time, we both got the same symbol as a result of our calculations–and that was kind of a giveaway. By this time, I had noticed that some symbols repeated, but for us to just happen to get the same symbol more than once was asking too much of chance.