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. You can buy my books here:
18 min - How Things Work - Steve Mould
More: How It Works documentaries