The classic demonstration illustrating Lenz's law by dropping a magnet through a copper pipe is presented using household aluminum foil right out of the box. Then comes the surprise. The teacher presents an aluminum foil cylinder with a missing lengthwise slice (cut before class). Will the demonstration still work? Students are amazed at the result, described in this paper and included in our accompanying video (Ruiz 2018 Video: Lenz's law with aluminum foil http://mjtruiz.com/ped/aluminum/).

A vacuum cleaner is used to isolate over twelve harmonics in a corrugated toy whistling tube. The toy tube is first taped along a horizontal surface. Then a vacuum cleaner with a hose diameter approximately the same as that for the toy tube is turned on. As the vacuum cleaner hose approaches one end of the corrugated tube, individual higher harmonics are generated. From a sound recording, the frequencies of the harmonics can be measured with the free audio program Audacity and found to agree with theory to about 1%. An exciting class demonstration is included with a video accompanying this paper: Ruiz M J 2020 Video: Vacuum cleaner isolates over 12 harmonics in a corrugated whistling tube (http://mjtruiz.com/ped/hummer/).

We would like to thank Catchatoor (2019 Phys. Educ. 54 016501) for discovering an error in the Villa reference (2016 https://youtu.be/ MEUufvpefEE) that we used in developing our model for a magnet falling down an aluminum tube with a slit (Berls and Ruiz 2018 Phys. Educ. 53 045001). We correct a sign error in the relevant figure and discuss the limitations of our model.