I am reading a textbook on special relativity and the author says at one point that he switches to natural units where c=1. The way he quickly explains this is by arguing that if c=3 x 10^8 m/s then we are working in units where seconds = 3 x 10^8m . . . . This has always baffled me and I never got a sufficient explanation of how we can just let c=1. Can anyone enlighten me?