OK folks, here's the secret ...
Initially I was very impressed but you need to realise that using the formula of subtracting the 2 numbers from your initial number is always only going to give you 9 potential numbers.
Any number in the 90s will give 81, in the 80s - 72, 60s - 63. Deduct 9 from each and you end up with
81,72,63,54,45,36,27,18 and 9. Try it with different numbers.
If you look at the list of shapes each of the above numbers has the same shape. The clever thing is that the shape against these numbers changes every time you click the Crystal Ball so your next calculation is immaterial the shape is there against every number before you click the ball.
So there it is, the secret's out.
Try it on kids especially, they think it is really magic!
Peter