Hi, I am learning SR and I need help to get the idea of relativity with two clocks. Yet I can understand that two different frames of reference can each one claim to be at rest, since this is just a logical argument. But I am not getting the point how they can each claim the other ones clock is the one who slows down, after all this is physical question and it is like two people arguing whether the earth is flat or round in which case only one can be right. To show an example of what bothers me, lets say that I and another person have synchronized clocks. Now when it is 12:00 on both of our clocks this person takes off in a linear motion and will never return. so when my clock will show 5:00, then if the other person is the one who moves then his clock will show 4:00, and if I am the one who moves then the other person's clock will show 6:00. So the person's clock can be either 4:00 or 6:00 but not both, yet we don't know what it is, but this is like if we don't know if the earth is flat or not and it is a physical question, and can have only one answer, even if we don't know what the answer is. It is clear to me that I am missing something, so what is it?