Nowadays our phones use GPS to let us work out where we are. But mobile phone networks were able to work out where you were long before phones got smart—and this video explains how.
It's all to do with base stations and some clever math which is used to send data to your phone. Crucially, aged as the system is, it means that even if you turn off location services on your phone, people can still work out (roughly) where you are. Worth remembering! [YouTube]