So I was recently reading Stephen Hawkings' "The Universe in a Nutshell" and came across the famous Twins Paradox thought experiment. My question is, since motion is relative, couldn't we extrapolate that either the observer on Earth is stationary and the rocket is traveling near light-speed (causing the clock on the rocket to run slower) or vice-versa (causing the clock on Earth to run slower)? If this is true, how can both clocks run slower than the other? I have read vague answers talking about how it has something to do with inertial reference frames, but what if no acceleration is involved? What if the rocket were to fly past Earth and mechanically flip a switch starting both clocks, and then doubling around and again hit the switch to stop both clocks? The clocks would then only be running while it is moving at a constant velocity. I'm sure I'm overlooking something, I just can't figure out what that is.