How to tell if a number is divisible by 9

This may sound dorky, but this is great trick I learned as a youngster, and sure to amaze a few friends.

Instructions

Difficulty: Easy

1. Like I said, this is a little dorky, but I have always thought this was pretty neat. I have also stumped a few friends on this trick as well . Tt has even won me money, or on occasion, a free beer as well. WIth the way gas prices or today, you could even bet a gallon of gas!
2. Using this knowledge, I can tell you whether or not a number is divisible by 9. In most cases, where the number is not very large with tons of digits, in my head.
3. If the sum of all the digits in the number add up to a number divisible by 9, then the number is divisible by 9. For instance, if someone asked you if 1521 is divisible by 9, you would have a 50/50 chance whether or not it is divisible by 9. Doing my trick, 1 + 5 + 2 + 1 = 9 means the number is divisible by 9. By the way, 1521 / 9 = 169. Pick a number, no matter how large, or small, and the sum of it's digits will be divisible by 9. There is no exception to this rule.
4. Good luck in stumping your friends into a free beer, or even a buck or two!