Kaprekar's constant

Watching some youtube last night and I came across a video about Kaprekar's constant, which is the number 6147. Amazing how people discover these facts (or why) . If you take the number 6147 and make the largest number possible from the 4 digits, which is 7641 and subtract the smallest number possible, which is 1467, you end up with 6147. so what I hear you say, it's just a fluke. Well, try the same with any 4 digit number, the only thing is that at least 1 digit must be different, when you get the answer, do the same with the answer, and eventually within a few moves, you will get 6147. Well I thought it was interesting, makes a change from the usual arguments.