Nuclear fusion
On Earth it is very difficult to start nuclear fusion reactions that release more energy than is needed to start the reaction.  The reason is that fusion reactions only happen at high temperature and pressure, like in the Sun, because both nuclei have a positive charge, and positive repels positive.  The only way to stop the repulsion is to make the nuclei hit each other at very high speeds.  They only do that at high pressure and temperature.  The only successful approach so far has been in nuclear weapons.  The hydrogen bomb uses an atomic (fission) bomb to start fusion reactions.  Scientists and engineers have been trying for decades to find a safe and working way of controlling and containing fusion reactions to generate electricity.  They still have many challenges to overcome before fusion power can be used as a clean source of energy.