  A part of relativistic dynamics (or mechanics) is axiomatized by simple and
purely geometrical axioms formulated within first-order logic. A geometrical
proof of the formula connecting relativistic and rest masses of bodies is
presented, leading up to a geometric explanation of Einstein's famous $E=mc^2$.
The connection of our geometrical axioms and the usual axioms on the
conservation of mass, momentum and four-momentum is also investigated.
