 What is the speed of the flashlight, given the fact that you have the speed of the rocket originally? Now, common sense says you have to add the two. You add the two, the velocity of one plus the velocity of the other is the net velocity. But you see in relativity theory, no, that's not true. Velocities do not add. And that's the key to the whole puzzle. So if you are in a rocket, traveling near the speed of light, and you throw a baseball...