Re: What is the only kind of energy that can be created or destroyed?

Energy can be neither created nor destroyed.  This principle is known as 
conservation of energy, see http://www.infoplease.com/ipd/A0384916.html .

So, until a flaw is found in the principle of conservation of energy, 
there is no kind of energy that can be created or destroyed.

Energy may change form, i.e. fission and fusion reactions convert mass to 
energy according to Einstein's equation

Energy is equal to mass times the speed of light squared,

but the mass so converted was already in an energy 'form'.  The mass of a 
uranium-235 atom, for instance, is said to have a rest-mass energy.

I hope this answers your question.

A good web site at which to begin your further study of conservation of 
energy is http://manta.library.colostate.edu/howto/energy.html .  Also, 
any physics text should include a discussion of conservation of energy.

I found the conservation of energy principle described on page 170 of 
Halliday and Resnick's 1966 Physics.