Check http://trunks.secondfoundation.org/files/psychic.swf out.
This trick is somewhat simpler than the previous number trick I explained. I'll explain how this one works.
Your original number can be written as 10a+b. Subtracting the sum of its digits (which is a+b) gives
Note that this is always a multiple of 9.
Observe that the images coresponding to multiples of 9 are all the same (with the exceptions of 90 and 99 because you can't get to these numbers because a is a single digit; it can't be 10 or 11).
Other numbers have this image too, and the images change each time, both things to make it appear more mysterious. The images also disappear at the end so one can't solve the mystery as easily.
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