>>It is possible. You have to divide the balls into 3 groups of four balls each. And then you should label each group A, B, and C. That gets you started.
>>
>
>I thought in this direction, but didn't come to a solution in case, the A and B don't weigh the same.
>
>>I've never heard of Great Ferma theorem. What is it?
>
>http://www.mbay.net/~cgd/flt/fltmain.htm

You have to weigh a combination of balls so that the odd ball is narrowed to within three possible balls.

Thanks for the link.