The paradox in the twin paradox is that the situation appears symmetrical so each twin should think the other has aged less, which is of course impossible.
There are a thousand explanations out there for why this doesn't happen, but they all end up saying something vague like it's because one twin is accelerating or you need general relativity to understand it.
Please will someone give a simple and definitive explanation for why both twins agree on which twin is younger when they meet for the second time?
