If you refer to the two digit number game, it is very simple:all two digit numbers from 10 to 99 have the peculiar mathematical property that when you add any of the two digits that form them, and substract the result from the original number, you will always end with a multiple of 9. As an example, let's take the number 35 , add 3+5=8, then substract 8 from 35, 35-8=27, which is 9x3. If you take, say, 57: 5+7=12, then 57-12=45, which is 9x5. In general, for all two digit numbers from 10 to 99 you will get 9, 18, 27, 36 and so on , until 81 (for numbers 90 to 99). The next thing they do is to assign the same symbol to those multiples of 9 , and mix them with other symbols assigned to the remaining numbers in order to disguise the trick. The effect is completed by changing the symbol they assign to these multiples of 9 in every new frame, every time you choose a new two digit number . Therefore, no matter what number you choose, they will always come with the right symbol (the one which is assigned to the multiples of nine in that particular frame), seeming to magically guess your thoughts!.