Try this with your friends: Take any three-figure number in which the first digit is the larger than the last...like 846. Now reverse it...648, and subtract the smaller from the larger making it 198. Now add that number to the same number reversed, 891. The answer is 1,089, and I didn't even have to use a calculator.

The answer will always be 1,089 even if the result of the subtraction is two figures, you just add a zero to the front before reversing. I can't be certain, but I would venture to say that the guy who figured this out, spent a lot of his youth wearing a pocket protector.

Source: READER'S DIGEST BOOK OF FACTS