Start with any natural number, such as 69534891. Count the number of even digits, the number of odd digits, and the total number of digits. In this case, there are three evens, five odds, and a total of eight digits. Use these three numbers as digits to form a new number: 358.
Repeat the steps with the new number, counting evens, odds, and the total number of digits. You get 123. If you perform the same set of operations on 123, you get 123 again.