One Coin That's Not Like The Others
RAY: Here's how you do it. You take the eight coins, and you make two piles of three and one pile of two. If the two piles of three balance, you put those aside because you know --
TOM: It's one of the other two coins!
RAY: Right. In the second weighing, you're going to put one coin in each side --
TOM: And it's going to tell you which one is heavier, immediately.
RAY: Right. Now, let's go back a step. If, in fact, the first time you put two piles of three on the scale, you get a result that shows that one side is heavier than the other, then you take those three heavier coins and put one on each side of the balance. If they are equal, then it's the coin that you didn't weigh, and if they're not then you know the answer.
TOM: Pretty darn nice.
RAY: So who's our winner, man?
TOM: The winner is Chanda, no last name given.
[ Car Talk Puzzler ]