On a table are two iron rods - one blue, one red. One of them is magnetic and one of them is not, they are otherwise identical.
Determine which is the magnet without using any other tools.
Place the bars so they form a “T” with the blue magnet as the top. A magnet’s field is strong at its poles, but nearly non-existent in the middle. If the bars stick, the red bar is a magnet, otherwise it is the blue bar.