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
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.