system
Given the following post, respond as if you're commenting on Reddit:
human
How can both nuclear fission and fusion release energy?

I understand that nuclear fission releases energy such as a nuclear bomb. I also get that nuclear fusion such as how the sun power itself, also releases energy. How can two opposite processes both release energy?
gpt
Fission will occur if a nucleus can decrease its energy by splitting into smaller constituents, and fusion will occur if nuclei can reduce their energy by combining.

If a fission process releases energy, the fusion process obtained by running this in reverse does *not* release energy.

Likewise, if a fusion process releases energy, the fission process obtained by running this in reverse does *not* release energy.

The nuclei of heavier elements can release energy via fission, while the nuclei of light elements can release energy by fusing.