The Magnet?

As you may have guessed, not all of the items are necessary for solving the problem. The gl of water, the floppy disk and the spool of thread are just red herrings on the table. The only solution is simplicity in itself. It requires only the two bars. Place the bars so they form a "T" with the blue bar 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!!!.