I really don't quite understand the result of time dilation. Let's say that we have two identical twins A and B. Twin A is at rest and twin B is on a high speed rocket ship. Lets say that 10 years has passed in frame A. According to relativity, the twin in frame B is supposed to measure less time. Let's say that only 1 year has passed in frame B. This is what happens if we consider frame A as the rest frame. However, just as an observer in frame A sees the rocket moving with nonzero speed, an observer in frame B (inside the rocket) will think that the rocket is standing still with frame A moving in the opposite direction. Shouldn't time slow down for frame A instead? This seems contradictory. How can you tell which frame will be the one that slows down or speeds up in time?