Nuclear fusion
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Fusion is the process of making a single heavy nucleus (part of an atom) from two lighter nuclei (atoms). The process, a nuclear reaction, releases a large amount of energy.
Although the nucleus that is created by fusion is heavier than either of the starting nuclei, it is not as heavy as the combined original weight of the starting nuclei (atoms). This lost mass is changed into lots of energy, as described by Einstein's famous E=mc2 equation.
Fusion occurs in the middle of stars like the Sun where hydrogen and then helium are fused together, releasing the energy which powers the heat and light of the star.
Not all elements can be joined; the heavier the element is, the less easily it can be joined with another. After fusion produces an atom of Iron (a metal) no energy is released in the reaction anymore. This is what causes stars to die. They join all of their atoms together to make heavier atoms of different types, until they start to make iron. The iron nucleus cannot fuse with other nuclei, and so the reactions stop, and the star eventually will cool down and die.
On Earth it is very hard to start nuclear fusion reactions that release more energy than is required to start the reaction. The only successful approach so far as being in nuclear weapons; the hydrogen bomb uses an atomic (fission) bomb in order to start fusion reactions. For over 50 years people have tried to make peaceful fusion reactions that can generate electricity, but so far this has not been successful. Many people hope that someday we will have fusion power for electricity, but no one is sure when that will happen, if it ever will.
