The Magnetic Rod
You are in a room with nothing else but two indistinguishable iron rods. You know that one of them is magnetized, how can you figure out which one?
SOLUTION
Touch the middle of the first rod with the end of the second rod. If the two rods get attracted to each other, then the second one is the magnet. If not, then the first one is the magnet.
Martin Gardner (1914 – 2010) was an American popular mathematics and popular science writer, with interests also encompassing scientific skepticism, micromagic, philosophy, religion, and literature—especially the writings of Lewis Carroll, L. Frank Baum, and G. K. Chesterton.