Alexander Bogomolny's "A Fake Among Eight Coins"
Problems about finding the counterfeit coin among a large group of otherwise genuine coins are quite abundant. Attempt to solve the problem on this webpage. Solutions appear at the bottom of the webpage. If this type of logical thinking interests you, attempt to find similar problems to solve with an online search.
Click http://www.cut-the-knot.org/blue/EightCoins.shtml link to open resource.