A mathematician demonstrating the "Kaprekar's Routine" and explaining why this number acts as a black hole for all others. In 1949 Indian mathematician D.R. Kaprekar introduced the world to a mysterious constant that now bears his name. Speaking at a conference in Madras, he revealed a strange property inherent in four-digit numbers. He showed that if you take any four-digit number (as long as it has at least two distinct digits), arrange the digits in descending order to form the largest number possible, then in ascending order to form the smallest, and subtract the smaller from the larger, a pattern emerges. If you repeat this process with the result, you will always reach the number 6174 within seven steps. Once you reach 6174, the process loops endlessly because 7641 - 1467 equals 6174.