So far so good. But now we hit a puzzle. If the motion of clocks is relative, then surely the time-dilation effect is also relative? Suppose we have two clocks, A and B, each in the lap of a human observer, moving relative to each other. In the reference frame of A, it is clock B that is moving, and hence slowed by time dilation. But in the reference frame of B, it is A that is moving, and therefore running slow. So each observer sees the other clock running slow! How can that be? It seems like a paradox. If A runs slow, it must fall behind clock B. But if B runs slow, A must gain relative to B. How can A be both behind and ahead of B at the same time? ...

     In fact, there is no paradox, as Einstein, who first raised the twins problem in passing in his 1905 paper, was quick to realize. The resolution comes from the fact that the two perspectives, Ann's and Betty's are actually not completely symmetric. To accomplish her trip, Betty must accelerate away from Earth, cruise at uniform speed for a while, then brake, turn around, accelerate again, cruise some more, and finally brake again to land on Earth. Ann merely remains immobile. All Betty's maneuvers, acceleration and deceleration, break the symmetry between the two sets of observations. The principle of relativity, remember, applies to uniform motion, not to accelerations. An acceleration is not relative; it is absolute. Taking this into account, it is Betty who ages less. On her return, Ann would be older.
     It is important to realize two things. First, the twins effect is a real effect, not just a thought experiment. Second, it has nothing to do with the effect of motion on the aging process. You must not imagine that the years spent in the rocket ship are somehow kinder to Betty on account of her confinement or movement through space.