The "Relativistic" Mug

This note is an attempt to explain in simple words why the famous relation $E=mc^2$ misrepresents the essence of Einstein's relativity theory. The note is addressed to high-school teachers, and a part of it - to those university professors who permit themselves to say that the mass of a body increases with its velocity or momentum and thus mislead the teachers and their students. ----- Contains both English and Russian versions.