The speed of light is the ultimate speed limit, even if we tend to perceive speed as a relative concept. For example, you are sitting here reading this text, and you might think you are still. But a friend of yours sitting in a train could say the opposite: for them, you do have a speed! However, no matter who or where you are: in vacuum, light always travels at the same speed. This simple and intriguing fact has tremendous repercussions for the evolution of our universe.