A space explorer A sets off at a steady $0\mathrm{.}95$c to a distant star. After exploring the star for a short time, he returns at the same speed and gets home after a total of $80$ years $($as measured by earth$-$bound observers$).$ How long do A$$s clocks say that he was gone and by how much has he aged as compared to his twin B who stayed behind on earth? This is the famous $$twin paradox$.$ It is fairly easy to get the right answer by judicious insertion of a factor of in the right place, but to understand it$,$ you need to recognize that it involves three inertial frames: the earth$-$bound frame S$,$ the frame S $$ of the outbound rocket, and the frame S $$ of the returning rocket. Write down the time dilation formula for the two halves of the journey and then add. Notice that the experiment is not symmetrical between the two twins. B stays at rest in the single inertial frame S$,$ but A occupies at least two different frames. This is what allows the result to be unsymmetrical.