I came across the number 6174, which is known as Kaprekar's Constant. What makes it so fascinating is that no matter which four-digit number you start with (as long as the digits are not all identical), the same process will eventually lead you to 6174.

The process is simple: arrange the digits from largest to smallest to form one number, then arrange them from smallest to largest to form another number. Subtract the smaller number from the larger one, and repeat the process with the result.

If you keep doing this, you will always arrive at 6174 in no more than seven steps. That's what makes it such a remarkable mathematical curiosity.