I have students bring two fairly strong ceramic magnets (but not supermagnets) together in opposition. When repelling you experience something that feels like an invisible putty ball between the magnets. I point out that there is a "sixth sense" that magnetotactic bacteria have* but humans do not. I also point out that when a compass needle turns, or if the magnets in their hand flip, work is being done so magnetic fields have energy and by Einstein's E=mcc magnetic fields must also have mass. So fields are like matter but not composed of atoms.
* See for example Magnetic Navigation in Bacteria, Scientific American, Dec. 1981.
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