Fake coins riddle: how do you identify them with a balance scale?

It depends on how many counterfeit coins there are and what information you have to start with, but the general trick is to divide the coins into groups and weigh them strategically to eliminate possibilities. If you know there's only one fake among several, you weigh them in groups: if the scale tips, the fake is on that side; if it balances, it's in the group you didn't weigh. With each weighing you eliminate options until you're left with a single suspicious coin or you confirm which one is heavier or lighter. The important thing is to use a logarithmic strategy, not weigh them one at a time.