How can this happen if each twin sees the other twin's processes running slow? The answer: one twin changes from his original moving inertial frame to another, totally different one (to make the return trip), which destroys the symmetry between the processes that would otherwise exist. The travelling twin is younger than the stay-behind twin when their frames coincide again.

In a practical case, the travelling twin has to change inertial frames three times... to take off, to turn around, and to land again. The other twin is always in the same inertial frame.

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