Good question. Let's take an example here on earth. Let's say we put Roger Clemens with his 90mph fastball on the back of a pickup truck traveling 60mph. When he hurls one of his fastballs, then is not his fastball traveling at 150mph relative to a person standing on the roadside? So then, if a spaceship traveling at the speed of light turns on its headlights, doesn't the beams of the headlights move at twice the speed of light relative to a stationary object and at the speed of light relative to the spaceship? Then again, maybe it doesn't work that way in space due to zero gravity. One thing for sure, I'd make a lousy physicist with this kind of logic!