Get 75% off NordVPN with this link: https://nordvpn.com/steve and use the promo code steve to get the first 30 days free. 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://www.youtube.com/watch?v=l8CI3bs9rvY The piezoelectricity video is here: https://www.youtube.com/watch?v=wcJXA8IqYl8 You can buy my books here: https://stevemould.com/books Patreon: https://www.patreon.com/stevemould Twitter: http://twitter.com/moulds Instagram: https://www.instagram.com/stevemouldscience/ Facebook: https://www.facebook.com/stevemouldscience/ Buy nerdy maths things: http://mathsgear.co.uk/