Does it mean that as you increase the energy of a particle, its mass will increase? Or, that for any given particle there is a certain container of certain mass/energy and part of that quantity is allocated to energy and the other to mass. In the latter case, could you not make a transistor out of this where you encode 0/1 states to certain mass/energy distributions? On an simple, intuitive level (I'm a physics layman): Why does it happen to be related by exactly the speed of light squared? How does this equation follow from the simple statement: The speed of light is the same in all inertial reference frames?