¥Simultaneity is
relative: events that occur simultaneously in one reference frame do not
appear to occur simultaneously in other, differently-moving, reference
frames.
¥Mass is relative:
an object seen in a moving reference frame appears to be more massive
than the same object seen at rest; its mass approaches infinity as the speed of
the reference frame approaches the speed of light. (This is why nothing can go faster than light.)
¥Mass and energy are
equivalent: Energy can play the role of mass, endowing inertia to objects,
exerting gravitational forces, etc. This is embodied in the famous
equation E=mc2.