I remember seeing a cartoon of Einstein at the blackboard on which the equation E=ma^2 (crossed out) then E=mb^2 (crossed out) and finally E=mc^2 with a beaming

I remember seeing a cartoon of Einstein at the blackboard on which the equation E=ma^2 (crossed out) then E=mb^2 (crossed out) and finally E=mc^2 with a beaming Einstein.