Here are two puzzles about finding fake coins among real ones, by weighing them.
The first puzzle uses a scale balance, and is easier than it first looks - how could you find the one fake coin out of 9 if the only way to do so was by weighing them on a balance scale? Oh, and you only get two weighings.
The second puzzle ups the challenge to locating the bogus bag of coins out of 9. Progress provides a weightier tool, in the form of an electronic balance that tells you how many grams something weighs. But when this puzzle gives with one hand, it takes with the other - you can only use the electronic balance once. Why? Just because, that's all.
0:10 - Puzzle 1 - Scale Balance
1:15 - Puzzle 2 - Electronic Balance
Solutions follow. The first puzzle really is worth sticking with, as it's not as hard as it first seems. The second puzzle is a bit of stinker, but has a perfectly logical reason once you accept you're only going to get one reading out of the electronic balance.