There are situations where you want to quickly change your mind and want to be distracted "spiritually" (an example is the self-fulfilling prophecy when an impending syncope, which can be broken by a quick distraction).
The task is to calculate the (checksum/iterative) checksum for a given number.
Example : 123456
Add all digits : [ 1 + 2 + 3 + 4 + 5 + 6 = 21 ]
If the result is not one digit , then add the digits again: [2 + 1 = 3]
Nothing special, but (in some cases) helpful...