Here's why:

* Atoms are made up of a nucleus (containing protons and neutrons) and electrons orbiting around it.

* Fusion involves the nuclei of two atoms merging to form a single, heavier nucleus.

* This process releases a tremendous amount of energy because some of the mass of the original atoms is converted into energy according to Einstein's famous equation E=mc².

This is how stars generate energy and create heavier elements from lighter ones. It's also the basis for research into fusion power, a potentially clean and sustainable source of energy.