Another oldie, but a goodie......
You're traveling through the woods and come to a bridge.....complete with the obligatory troll. The troll tells you that not only do you have to answer his riddle to cross the bridge, if you fail, he's going to eat you. The good news is that if you answer correctly, not only may you cross his bridge, you'll also leave with wealth beyond your wildest dreams.
He points to 5 large chests, each filled with gold bars. All the bars are visually identical, but the troll tells you that 4 of the chests contain fake bars and only one contains real gold. The ONLY difference between the bars is that while each fake bar weighs exactly 10 lbs, each real gold bar weighs exactly 10 lbs, 1 oz. Your challenge is to choose the chest with the real bars. Do so and you'll be allowed to take it with you as you continue your journey. Fail, and you get eaten.
The individual bars are far too heavy to feel the weight difference, but the troll offers you a scale, accurate to the ounce, to help you choose. Unfortunately, it's an old "penny" type scale that only gives you a single weight printed on a slip of paper, and the troll informs you that you're only allowed to use the scale one time.
With only one use of the scale, how do you determine which of the 5 chests contains the real gold?