I liked this short description of the constancy of the speed of light from Scott Adam's DNRC Newsletter #26 (October 1999):
Did you know that if you got in a rocket ship and raced a beam of light, the light would always be faster than you by exactly the speed of light, no matter how fast you went? It sounds like a crackpot idea but it's Einstein's theory. Most scientists agree that Einstein is right even though it makes no sense.