Pick any four-digit number whose digits are not all the same. Sort its digits largest first, then smallest first, and take the small one from the big one. Do the same to the answer, keeping a leading zero if one turns up. Within seven goes you will reach 6174, and after that it only hands you 6174 back: 7641 take away 1467. D. R. Kaprekar found it in 1949, and what I like is that 6174 is not interesting for anything it is. It is interesting only because of where everything else ends up, like the low corner of a field that all the rain finds.