Space Travelled Twin Sibling Younger Than the Other

My physics teacher asked us to find a solution for this............
When there are twin siblings and one of them is travelling to space. When he returns to the earth the one who travelled to space will be one year younger than the other ............How and why ??????
Somebody explain me the reason..............
When there are twin siblings and one of them is travelling to space. When he returns to the earth the one who travelled to space will be one year younger than the other ............How and why ??????



Somebody explain me the reason..............