Step 1: Idea behind both processes.
Both fission and fusion are ways of reaching a more stable nucleus, that is a nucleus with a higher binding energy per nucleon. Iron (mass number about 56) sits at the peak of the binding-energy-per-nucleon curve, so nuclei move toward it to release energy.
Step 2: Fission described.
Very heavy nuclei lie on the right of that peak. If such a heavy nucleus is broken into two medium-sized nuclei, the fragments have a higher binding energy per nucleon than the parent, so energy is set free. This breaking of a heavy nucleus into two comparable parts (triggered by a slow neutron) is called nuclear fission. A representative reaction is \(^{235}_{92}U + ^{1}_{0}n \rightarrow ^{144}_{56}Ba + ^{89}_{36}Kr + 3\,^{1}_{0}n\), liberating nearly 200 MeV and extra neutrons that sustain a chain reaction.
Step 3: Fusion described.
Very light nuclei lie on the left of the peak. If two of them are joined into a slightly heavier nucleus, again the binding energy per nucleon rises and energy is released. This joining of light nuclei is called nuclear fusion. A representative reaction is the deuterium-deuterium reaction \(^{2}_{1}H + ^{2}_{1}H \rightarrow ^{3}_{2}He + ^{1}_{0}n + 3.27\) MeV, and inside stars four protons ultimately fuse into one helium nucleus.
Step 4: Why fusion is hard to start.
Fusion requires temperatures of the order of millions of kelvin so that the positively charged nuclei have enough kinetic energy to come close enough for the strong nuclear force to bind them, overcoming Coulomb repulsion. Fission needs only a slow neutron, which carries no charge and is not repelled.
Step 5: Common conclusion.
In each case a small mass difference between reactants and products is the true fuel, released as energy through \(E = \Delta m c^{2}\).
\[\boxed{\text{Both raise binding energy per nucleon and release energy}}\]