A rule of general physics that states that energy cannot be created or destroyed, only transfered. This law also applies to matter and is called (inventively enough) conservation of matter.

Energy cannot be created or destroyed.  If you have a reaction in a closed system, the total energy at the beginning will be the total energy at the end.  Some individual things may have more energy, but that means that other things will have less.  In any standard chemical reaction (involving the outer-shell valence electrons and not the nucleus of the atom), this law holds perfectly steady.

Nuclear reactions seem to create energy from nowhere.  However, this is not so if energy and mass are considered as equivalent - using Einstein's famous equation, e=mc^2;  The Energy is equal to the mass times the speed of light squared.  The speed of light is a huge number (3.00*10^8 m/s), and it becomes even more monstrous if you square it.  Thus, a small amount of mass (Less than 1.66*10^-27 kg, 1 atomic mass unit) can be converted into huge amounts of energy.  The mass seems to be destroyed, and the energy seems to be created!

However, since mass and energy are the same thing, the Law still holds.  Now, however, it can be known as the conservation of mass-energy.    This is the basis behind the atomic bomb and nuclear reactors.