Flash Mind Reader Solution
Thu, Feb 13, 2003; by Dave Winer.
Here's an explanation of how the Flash Mind Reader works. If you want to puzzle it out for yourself, read no further. You have been warned!
A false path. At one point we thought it was tracking the mouse. Haha. Except it isn't. Even if you don't touch the mouse amazingly it always gets the right answer. Hehe.
Tim Bray was the first to respond, and he provides half the solution, because there are two tricks at work here, not just one. If you do the math, you'll see that every possible result has to be a multiple of nine. Any two-digit number can be expressed as a*10 + b. For 73 a=7, b=3. Now the puzzler requests that we subtract the sum of a and b from the number, a*10 + b - (a + b), which becomes a * 9 if you reduce it. Therefore it's a multiple of nine.
Daniel Berlinger supplies the second half of the solution by noting that the glyphs of all the multiples of nine are the same. They are? They are. That's the trick! Every time we refresh the screen the symbols change, unbeknownst to us. It gets better. 90 and 99 which are multiples of nine, have different symbols from the other multiples of nine. But there's no way to get those numbers to come out the other end of the algorithm. Nice twist.
If you liked the Flash Mind Reader, rip your hair out over this one.