Your OP stated one is lighter, not one is different. It's easier than the 12-coin one different puzzle, but introduces the ternary concept well enough to get y

Your OP stated one is lighter, not one is different. It's easier than the 12-coin one different puzzle, but introduces the ternary concept well enough to get you started on the harder problem (there are some other complications to it as well). Weigh 2 coins. If the scales move you've found the lighter coin. If the scales balance, it's the third coin you didn't weigh.