Okay, so I am reading a book, "The Elegant Universe" by Brian Greene, which talks about motion and its effect on time.
Greene makes the point that time changes with motion by saying that if you have two mirrors and bounce a photon off of them it will have bounced off them very very often during a second. But, if you have the mirrors moving you must shoot the photon at an angle so it will be able to hit the mirrors and not miss and fly off. Since you are shooting diagonally it takes the photon more time to hit each mirror. So it's basically saying that time changes the closer you come to light speed.
Why? Why does this hold true for all matter? How does bouncing a photon around prove anything?