In the twin paradox, the twin who has gone into space and back at relativistic speeds supposedly comes back younger than the one who stayed on earth, the reason being that from the earth twin's reference frame, time has gone more slowly for space twin. However, what has puzzled me is that from space twin's reference frame, time has gone slower for earth twin, so presumably when space twin returns, it will be space twin who is the younger. Who is younger upon space twin's return? Could somebody explain why there is this paradox - in fact, is this the twin paradox, in which case reply with the answer 'yes' and ignore this thread thereafter. But if not, what is wrong with my reasoning?