I then added some intrigue/obfuscation by hiding a stack of potent neodymium magnets inside a hollowed out red whiteboard marker. I told students that the tube could detect the marker's color and try to stop the fall of the red marker. To "prove" my point, I would drop an unmodified green marker through the tube showing that the tube offered no resistance. Some students would demand up-close inspection. They found the red marker was heavier: the heavier marker fell more slowly!
I typically ran this demo on the first day of school as part of my "Physics Begins with an M" first day of school lesson. The explanation had to wait until second semester, when we studied magnetism.
Since this demo was so costly, I felt I had to add value. I found I could balance it vertically on my classroom floor to demonstrate unstable equilibrium. And when it toppled over, the sound it made could pierce your soul. Students would emit involuntary screams.
Holding the tube with thumb and forefinger at various places while gently bouncing it on the tile floor produced different notes as different resonance modes were activated.