OT - Interesting Mind Teaser

[Gramps]

Ok, my friend sprung me on this tonight when (a) I was already semi-drunk (on the way to being fully drunk), and (b) without a pen and pad in front of me to help figure it out. He works with some Stat PhDs who decided to use this quesiton when interviewing other Stat PhDs who they were thinking of hiring.

I love these mind teaser things, kind of like an LSAT Logic Game type of thing (on steroids), so I couldn't get it off my mind (I was sitting at the SF Giants vs. Atlanta Braves game using my fingers to try and figure it out). I finally got if after a few innings and about 7 incorrect answers. If you know it already, keep quiet - no posting the answer to make you look smart if you've had prior help. If not, enjoy (if you like these sort of things)...

...You have 12 stones. 1 of the 12 weighs more or less than the other 12. You have to figure out which one it is, and whether it weighs more or less. How to figure this out? You have a balancing scale, and can only use it 3 times. How do you get the above task done?