system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is it that fission and fusion both create enormous amounts of energy when they seem to be the exact opposite processes? (i.e. splitting or combining atoms)
gpt
For the same atoms, they don't. You're correct that undoing a fission reaction takes as much energy as it would release.

But they're not done on the same atoms. The binding energy of nuclei (basically the energy you'd need to rip them apart = the energy you gain by putting them together) peaks around iron and nickel. For smaller atoms than that, fusion produces energy and fission consumes it. For larger atoms, fusion consumes energy and fission produces it. 

Most fusion (both in stars and for power generation here on Earth) fuses hydrogen, the lightest element, into helium. Fission takes place spontaneously (i.e. without you doing anything to the nucleus) only in the heaviest elements.