For a few minutes diversion, try this online card trick.
Amazing, eh? Just how can the computer read your mind?
Below the fold is my explanation. Don't click unless you are really stuck.
Lets say we pick a two digit number where the first digit is x and the second digit is y.
So the number we just picked is (10x + y).
Now we have to add x and y and subtract it from the number we picked:
(10x + y) - (x + y)
10x + y - x - y
The y's cancel each other out which leaves
In other words, for any two digit number you pick, the resulting number will always be 9 times the first digit, and the second digit is irrelevant.
Now look again at the table of symbols. Did you notice how the symbols for all the multiples of 9 are identical? This way the computer can "guess" it every single time - there's no way it can get it wrong.
Oh yes, and just to make it more interesting, the computer changes the symbol that it associates with the 9s each time. One game it's a star, next game it's a cresent (or whatever). To prove it to yourself, look at the symbol for number 9 and click the button - it will always match.
Did you guess it?
See also this classic card trick.