Something sparked my interest yesterday, so I started watching videos and reading about relativity. (This seems to happen every year or so.) I get hung up on the same thing every time! Many discussions I've seen use the example of an astronaut that takes a trip into space for 10 years or so and then comes back to see that his twin has aged significantly more than he has. If velocity is relative to each observer (correct me if I'm wrong), why do these examples always treat the people on earth as being stationary? To the astronaut, wouldn't it seem as though his twin on earth was the one who had been traveling at a high velocity over the course of the previous 10 years, and thus be the one that has aged slower? My brain is about to explode. Please help!