In the documentary the Fabric of the Cosmos Leonard Susskind remarks how confused he is that a photon could be both a wave and a particle, he says a rock is a rock, and a wave is a wave (a picture of an ocean wave crashing). How could a rock be a wave? To me the answer is obvious, a wave is just an abstract description of how matter moves. Any material body can be both a body and a wave so long as it moves in a wave form. I don't understand why this is hard to understand. I put this in the classical category because it concerns optics.