You and a friend synchronies your watches. Your friend gets in a rocket ship and blasts off. He manages to get the ship going to 90% speed of light. You use a telescope to check out his watch. To you it looks like it’s running slow. When he lands back on earth you check both the watches and see that his is behind yours. We’ve probably all seen a version of this story before. We’ve also probably all heard of the experiment that proved this effect (jet with the atomic clocks). The only problem I have with it would be if you looked at it from the ‘friends’ point of view. Wouldn’t it be true that if your friend on the spaceship decided to look at your clock with his telescope, he would see yours running slow? If he kept watching it, he would see it become really behind his. Yet when he lands, somehow his is the one that’s all of a sudden behind? What am I missing?