If i have two fish in a fish tank say fish A and fish B and the owner of the tank is looking into the tank each fish thinks he is travelling at some velocity. fish A looks at fish B and Fish A doest know if he is moving or if fish B is moving. Same for fish B. But the owner looking into the tank can absolutely tell which fish is moving. As I said in another post two frames of reference are always conatined within a third larger frame. can someone explain to me why the perspective of the fish is the preferred frame? the owner measures the fishs velocity and call him on this two way radio and tells him his velocity. Is this an absolute velocity?