Write down a three-digit number whose first and last digits differ by at least two, say 732. Reverse it and subtract the smaller from the larger: 732 minus 237 is 495. Reverse that and add: 495 plus 594 is 1089. Start with any number you like and you land on 1089, which is why it turns up in magic tricks dressed as mind-reading. The reason is that the subtraction throws away almost everything you chose. The middle digit cancels at once, and what survives is ninety-nine times the gap between the outer two: always a number with a nine in the middle and outer digits that add up to nine. The trick feels as if it is reading your number. It stopped listening after the first step.