Take any four digit number, follow these steps, and you’ll end up with 6174.

1. Choose a four digit number (the only condition is that it has at least two different digits).

2. Arrange the digits of the four digit number in descending then ascending order. 

3. Subtract the smaller number from the bigger one. 

4. Repeat. 

Eventually you’ll end up at 6174, which is known as Kaprekar’s constant. If you then repeat the process you’ll just keep getting 6174 over and over again.