You have two bars of iron.

physics/magnets

Problem

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.

Show solution

Take the two bars, and put them together like a T, so that one bisects the
other.
                       ___________________
          bar A --->  |___________________|
                               | |
                               | |
                               | |
                               | |
          bar B ------------>  | |
                               | |
                               | |
                               |_|

If they stick together, then bar B is the magnet.  If they don't, bar A is
the magnet. (reasoning follows)

Bar magnets are "dead" in their centers (i.e., there is no magnetic force,
since the two poles cancel out).  So, if bar A is the magnet, then bar B
won't stick to its center.

However, bar magnets are quite "alive" at their edges (i.e., the magnetic
force is concentrated).  So, if bar B is the magnet, then bar A will stick
nicely to its end.