Here's why:

* Fusion is the process where two or more atomic nuclei combine to form a heavier nucleus, releasing a tremendous amount of energy. This is the process that powers the Sun.

* Fission is the process where a heavy atomic nucleus is split into two or more lighter nuclei, also releasing energy. This is the process used in nuclear power plants.

In the Sun, hydrogen nuclei (protons) fuse together to form helium nuclei, releasing energy in the form of light and heat. This process is what makes the Sun shine.