In Einstein's own words:

**
***"It followed from the
special theory of relativity that
mass and
energy
are both but different manifestations of the same thing -- a somewhat unfamiliar
conception for the average mind.**
*
*Furthermore, the equation
E is equal to mc-squared, in which
energy is put equal
to
mass, multiplied by the square of the velocity of
light, showed that very
small amounts of
mass may be
converted into a very large amount of
energy and
vice versa.** *
*The
mass and
energy were
in fact equivalent, according to the formula mentioned above.**
*
*This was demonstrated by Cockcroft and Walton in 1932, experimentally."*