You have got twelve (12) coins. They are all identical except for one
which has a different weight, meaning one coin is slightly lighter OR
heavier.

You use a balance...that thing with two "arms" where you can put
weights on both sides and see wich side is heavier.
You can choose any combination of coins and weigh three (3) times.
After weighing three times, you have to be able to tell which coin is
the one with the different weight.

Hi m00se!
Here's a link to a possible solution to this wonderful puzzle (I think
it's one of the best "weighing" puzzles I know of). As you'll see, the
solution can be quite difficult.

Scale Puzzles 301
http://www.mazes.com/scales/Scale301.html

And, in case you're interested, here's a link to a solution to an even
harder version of the same puzzle.

The Twelve-Coins Puzzle
http://www.av8n.com/physics/twelve-coins.htm


Google search terms:
balance "twelve coins" puzzle
://www.google.com.ar/search?hl=es&q=balance+%22three+coins%22+%22twelve+coins%22&meta=


I hope this helps!

Best wishes,
elmarto