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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:    • How Do Atomic Clocks Work?  

The piezoelectricity video is here:    • Piezoelectricity - why hitting crysta...  

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