This article is from the Puzzles FAQ, by Chris Cole firstname.lastname@example.org and Matthew Daly email@example.com with numerous contributions by others.
You have two bars of iron. One is magnetized along its length, the
other is not. Without using any other instrument (thread, filings,
other magnets, etc.), find out which is which.
Take the two bars, and put them together like a T, so that one bisects the
___________________ bar A ---> |___________________| | | | | | | | | bar B ------------> | | | | | | |_|