6174 & Kaprekar's Operation

Take any four digit number, rearrange the digits so you have the largest possible number and the smallest. Subtract the smallest from the largest. Take the result, and likewise rearrange the digits to get the largest and smallest numbers. Again subtract the smallest from the largest. Continue doing so. At the most, but the seventh iteration of this operation, you will get the number 6174. If you rearrange 6174 to the largest and smallest and repeat the operation, you will get 6174. It doesn't matter which four digit number you start with, you will always end with 6174.

5200 - 0025 = 5175
7551 - 1557 = 5994
9954 - 4599 = 5355
5553 - 3555 = 1998
9981 - 1899 = 8082
8820 - 0288 = 8532
8532 - 2358 = 6174
7641 - 1467 = 6174

How weird is that? This was discovered by D.R. Kaprekar, an Indian mathematician in 1949.