Can you solve the MAGNET RIDDLE? ft YouTube CEO Susan Wojcicki

For the metal bars:
With a string:
Tie the string around one metal bar and see if it is affected by earth's magnetic field
With a mallet:
By hitting the metal very hard, you can both add and remove a magnetic field from iron (I believe "Magnetism in iron as a function of pressure" by Gerd Steinle-Neumann, R E Cohen and Lars Stixrude is this effect)
With the file:
File off some metal off both bars into 4 piles (2 of each) and test each bar against one of each bar.