Have you ever heard of such a thing as a dimensionless constant? You could in principle have any dimensionless number k and E=mkc2, but that would not not work with the definition of relativistic momenta as seen in the link posted in response #26. k must therefore be equal to one. Are you trying to confuse the kid?What else could it be? There is no other way to get a constant with units of [itex]L^2/T^2[/itex] from any of the other universal constants. So it has to be some multiple of [itex]c^2[/itex]. The only other possibility would be for there to be no conversion between mass and energy.