Today I found a mind reader website called MysticalBall(en) or 读心术(cn). This site is a flash game.
In this game, each number in 0-99 maps to a symbol in a table. And the game works like this:

Choose any two digit number, add together both digits then subtract the total From your original number*
When you have the final number look it up on the chart and find the relevant symbol. Concentrate on the symbol and when you have it clearly in your mind click on the crystal ball and it will show you the symbol you are thinking of …

*For example if you choose 23: 2+3=5. 23 minus 5 will give you your answer.

It’s amazing, huh? Now we will see why this works.

First, you need to use a radom two digit numbers, we assume that they are a and b. So the sum of the two number is a+b, and the digit [ab] we write like 10 multiplus a add b (10×a+b). Now we have the euqation (10×a+b)-(a+b)=10×a+b-a-b=9×a. a is an integer, so 9×a will always be the multiple of nine.

Now we go back to see the table. We can find that the numbers which are multiple of nine (like 18, 27, 36, 45, 54, 63, 72, 81, 90, …) belong to the same symbol. Althrough the symbol will change between difference times. When you play more than once, you will find the symbol is different. But the multiples of nine is always the same.

So that’s why this game works even you didn’t tell the website any words.

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中文概述。[不要把我误会成翻译，上面的也是我的原创。]

让我来说一下原理。首先，我们设两个数为a, b。所以，两者和是a+b，而两位数[ab]可以表示成10×a+b，所以，我们就有了等式(10×a+b)-(a+b)=10×a+b-a-b=9×a. a是整数，所以9×a就总是9的倍数的数字：18,27,36,…

再仔细看那个看似杂乱无序的表就都明白了。所有9的倍数所对应的符号都是一样的。当然，为了更能让人相信，所以每次玩过之后，图案都会随机改变，9的倍数的图案也会改变，但他们是相对不变的。所以，你怎么想，它都只要举出9的倍数的那个图案就可以了。这就是为什么，你什么都不用跟电脑讲，它就懂的道理。

很简单的小把戏，不过居然无数人都被骗过。