3 the magic number

The logic may be quite familiar to every one but its worth a relook.

Divisibility test for 3:
To check a number whether it is divisible by 3 or not, add all the digits of that number to get a number which is the sum of the all the digits. If the resulting number is divisible by 3 then the actual number is will be divisible by 3.

The same logic is applicable for divisibility test of 9.