An instructor is a person who has answers for the questions rised by his students. Consider that one of them comes with the following one: If I am co-moving with a tardyon then I measure its rest mass (energy) obtaining a zero value for its momentum. Nobody can comove with a photon and so nobody can measure its rest mass and nobody can say that its momentum is equal to zero. Can I conclude that the magnitude of a physical quantity that I am not able to measure is equal to zero. I have not so far an answer with pedagogical benefits. Have you one?