12 coins

This is a real tricky logic puzzle that I found in the 'bonus' section of this site. I was quite amazed that the following is possible, even after deducing the solution with some pencil and paper. This is the gist of the puzzle.

You have 12 coins, each identical in size and shape. One of these coins differs in weight from the rest. You have a scale, and must determine which coin does not the weigh the same and whether that coin is heavier or lighter than the rest. The challenge is to reach this conclusion by making at most three weighings.

To elaborate on the puzzle requirements I will formalize the idea of weighings. The scale is a machine to compare the summed weight of two disjoint subsets of the 12 coins; a single comparison of subsets classifies a weighing. The exact output of the scale is not specified in the problem, so you can probably use any kind of (reasonable) scale output you need.

I will work on turning my solution into a post and getting that out in a few days.

There is a similar and much simpler problem on the same website here.

Hope this one catches your interest as it did mine. Enjoy ^.^