I don't know if this has been posted already, but anwho... If you pick any positive integer greater than 9 and subtract the sum of its digits from that number, you'll end up with a multiple of 9. How do I know it's a multiple of 9? Curiously enough, the sum of the digits of a positive integer that is a multiple of 9 is a multiple of 9. Do this enough times and you'll end up with 9. I tried this with other number systems and it seems to hold. This is the most interesting play of numbers if found to date.