Why is the sky blue?
What would happen if you turned on your headlights while moving at the speed of light?
Albert Einstein figured out that the speed of light is the one true constant in the universe (unless you count the fact that gravity gets stronger as you get older). That means that wherever you are, however fast you're traveling, light always moves at the speed of light.
Let's say you managed to crank that ol' Vee-eight Ford until you were leaving Earth at the speed of light (that's impossible, but we'll just ignore that thorny detail). When you powered on the high beams, their light would streak away from you at the speed of light. It seems paradoxical, but when seen from Earth, their beams would also be going at the speed of light.
The answer to this paradox lies in the curious geometry of space-time, as described by Einstein and his intellectual heirs. Not being among them, we'll just mention that understanding this kind of stuff -- or at least claiming to understand it -- is what separates physicists from us mortals.
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