Dumb answer: because it emits light (electromagnetic radiation).
What is light? A: A wave of electricity and magnetism. Good discussion on p92
of textbook. We can specify waves by a number of properties; two of the most important
are wavelength 
and speed at which the wave propagates.
Two other important ones are the amplitude of the wave and the frequency.
Demonstration with wave machine.
For light waves the speed is special 
meters/sec =
186,000 miles per second.