Number "mind reading" game

So, check out this site Wizardy - mind reader | milaadesign.com | online flash games | virtual tour provider Now, how does this work? I’ve tried it a few times, and even though the symbols have been different, it has chosen correctly each time. I’m sure there’s some simple mathematical explanation, but it is kind of creepy …

Adding all numbers together and subtracting that sum from the original number will end in a multiple of 9. If you look carefully, you will see all multiples of nine are the same symbol.

Thanks! You’re absolutely right. I knew it had to be something simple like that. If you pick “play again”, it swaps the symbol that is by the multiples of nine, so it’s not the same symbol twice in a row.