Fission and Fusion are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It suggests what word you may want to save time, but quite often, its suggestions couldn’t be more off base and produces humorous results.
Fission is a noun, it means the splitting of objects. In science, fission of atoms in the lab to learn more about their contents and energy.
Fusion is a noun, it means the process of combining. In physics, nuclei experience fusion. Fusion can also describe a unique combinations things: food styles, music styles, or political ideologies.
The following story uses both words correctly:
Fuschia was fascinated by energy and sciences, including nuclear fusion and biological fission.