Any four-digit number, one routine, and it always lands on the same number
Pick any four-digit number that isn't made of all the same digit, say 3524. Arrange its digits into the largest possible number and the smallest possible number, then subtract the smaller from the larger. Repeat the process on the result. What happens if you keep going?
Reveal the answer
You always reach 6174 within seven steps, and once you're there the routine just produces 6174 again forever (7641 − 1467 = 6174). D. R. Kaprekar discovered this in 1949, and 6174 is now called Kaprekar's constant; the equivalent routine for three-digit numbers always lands on 495 instead.