After a fairly long time of thinking I had a decent grip on the concept of time dilation, it has suddenly occured to me that I don't. My issue is with the concept of the aging twins thought experiment (or whatever it's called) where if one shoots off at some comparable speed to the speed of light they will come back and be relatively younger than the twin that didn't move. I will quote from wikipedia: "Time dilation would make it possible for passengers in a fast-moving vehicle to travel further into the future while aging very little, in that their great speed slows down the rate of passage of on-board time." My issue with this is that relative to each twin, they themselves do not move at all. I.e. Each will observe the other to move slower through time than themselves. So then, if one twin shoots off at, say, 0.9c, there will be significant time dilation, and they will observe their twin staying young while they themselves age. My question is, how is it possible for both twins to witness each other age slower than themselves? In other words, who would be older when the twin came back? I've read through many threads like this in the past and nodded my head thinking I understood, but now it seems apparent that I don't.