Write the number 1089 on a piece of paper and stick in it your pocket.
Give a person a piece of paper, a writing instrument and a calculator. Instruct him to write down a three-digit number that consists of three different numerals. For example, the number 154 is acceptable, while 155 is not.
Tell the person to then reverse the digits in the number and write it down. For instance, if the person wrote down 429, he should write 924. Instruct him that the larger number should appear above the lower number.
Ask him to use the calculator to subtract the lower number from the higher number. He should write that number underneath the other two numbers.
Instruct him to then reverse the numbers of his three-digit answer and add it to the answer. For example, if his answer was 495, he should add 594 to it. Have him punch those numbers into the calculator and write down his answer.
Take the piece of paper out of your pocket and show him the number 1089 that you had written down. This will be an exact match to his answer.