Gold and Nickel
You have 15 identical coins – 2 of them made of pure gold and the other 13 made of nickel (covered with a thin gold layer to mislead you). You also have a gold detector, with which you can detect if in any group of coins, there is at least one gold coin or not. How can you find the pure gold coins with only 7 uses of the detector?
Puzzle from the archives.