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Quartz watches have a tiny crystal tuning fork inside that vibrates at 2^15 Hz and there's a really clever reason for that. This video also talks a bit about how mechanical watches work.
CORRECTION: Ok, so I actually hung 16 flip flops! So you do need 15. I'm an idiot.
The atomic clock video is here: https://youtu.be/l8CI3bs9rvY
The piezoelectricity video is here: https://youtu.be/wcJXA8IqYl8
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