Einstein postulates that the light travels at the same speed $c$ in every inertial frame of reference.
Now, if I'm within a train which goes right with velocity $V$ and throw a ball to the right with velocity $v_1$, the ball's velocity on my reference frame would be $v_1$. And from outside the train its velocity would be $V + v_1$.
Now what if (from inside that same train) I turn a lantern on to the front of the train. How come the photon doesn't have velocity $V + c$? What is being "sacrificed" in order to keep the photon's velocity equal to $c$?
