Answers:
Fission breaks apart nuclei
Fusion puts them together.
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Explanation:
The term "fission" is related to the term "fissure" which means "a cut or divide", often describing a geologic feature of the earth (eg: a crack in the rock).
Fusion is the idea of putting things together, which is the opposite of fission. The term doesn't have to apply to nuclear fusion. It could be something like the fusing of two ideas to create one new idea.