In 15-4,Feynman tried to explain why the rod perpendicular to the motion does not shrink."How do we know that perpendicular lengths do not change? The men can agree to make marks on each other's y-meter stick as they pass each other.By symmetry, the two marks must come at the same y- and y'- coordinates,since otherwise when they come together to compare results,one mark will be above or below the other,and we could tell who was really moving." The last sentence means that we can not tell which system is moving by doing experiments except looking outside.But is comparing the y'- with y- a kind of "looking outside"?? As y- is lying outside actually. So why?