You can also start with 5 balls on each side, leaving 2 aside to measure if the others weigh the same.
Ok then. Here's the same riddle with a nasty twist:
You have 12 similar looking balls. 11 of them weigh the same. One of them has a slightly different weight, BUT you don't know whether it is heavier or lighter. You also have a scale. You can put balls on both sides of the scale and it'll tell you which side is heavier or will stay in the middle if both sides weight the same. Again, you can use the scale only three times. How can you find out whether the different ball is lighter or heavier and which one it is?
(I haven't been able to figure this one out, but it is solvable as far as I know.)
