I know very little about relativity. I have heard that it implies that----- if A and B be two tweens. If A goes in a space trip and then return to B at earth, he will be younger than B. What I am confused is, It must be same thing whether A went away from B in his space trip on the ship or B went away from A ( in B's earth-Ship!!). Then how can it be that A Is younger. It will be impossible I think to determine who went for the space-trip and who didn't. Please feel free to ask for clarification if you don't get me.