system
Given the following post, respond as if you're commenting on Reddit:
human
Why Do Both Fission & Fusion Release Energy?

My thinking is that they're opposites so surely they can't *both* release energy. I'm obviously wrong but I don't know why.
gpt
Fission and fusion on the *same* nucleus would not both release energy; for example, you can release energy by fusing hydrogen into helium, but not by fissioning helium into hydrogen. Some nuclei release energy when they fuse and some release energy when they split. It depends on how tightly packed the nucleus is relative to the products of the interaction; the more tightly packed a nucleus is, the more energy it can release to get to that tightly packed state. Approximately speaking, you can get energy out of the nucleus if you fuse two light nuclei together into a heavier one that is not more massive than nickel/iron. Above that fission is what releases energy. The switch happens because the nuclei get more and more tightly bound (relative to their mass) until you get to nickel/iron, and then start to get less and less tightly bound relative to their mass as you go higher up in atomic mass.