Logic Puzzle

Twelve billiard balls all weigh the same, except for one that is either light or heavy. Determine in three weighings on a balance scale which is the "odd ball", and whether it is light or heavy.

Answer to the last Logic Puzzle:

Start 3 of the ends burning at the same time. When one rope (the one whose both ends were started burning) is completely burned out (30 minutes), light the unburned end of the other rope. By the time that rope is burned, 45 minutes will have passed.