The answers here are mostly correct, however it's important to note that the speed of light is only c in a vacuum. In a typical coaxial cable, for example, the speed of electromagnetic propagation is roughly 0.75c. Still incredibly fast, but not the same.

With a pulse generator, an oscilloscope, and a suitably long coaxial cable, you can setup a demonstration and see this effect. A single short pulse will reflect at the unterminated end of the cable, and by measuring the time difference between the emitted and reflected pulses you can determine the speed of electromagnetic propagation in the cable.