I just heard this, and only checked it for this one case.

1) Write down any 4-digit number*
2) Under that line write the digits from greatest to smallest
3) Under that line write the digits from smallest to largest
4) Subtract line 3 from line 2, this is your new line 1
5) Repeat 2 through 4 as necessary

You will eventually come to the number 6174, which always returns itself. Is this explained anywhere? Are there other numbers for 5-digits?

E.g.
1) 1584
2) 8541
3) 1458
4) 7083

2) 8730
3) 0378
4) 8352

2) 8532
3) 2358
4) 6174

2) 7641
3) 1467
4) 6174


*There are exactly 10 fails to this, and it's when all 4 digits are the same number. E.g. 1111,2222,etc.