system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did they know splitting the atom, fission, would release so much energy? And why would the opposite be also true, fusion?
gpt
Small atoms release energy when they combine, big atoms release energy when they break, and both gravitate towards 'medium' size. That 'medium' size is the iron atom.

Fusing large atoms or splitting small ones does not release energy, but actually absorbs energy.

Scientists studied radioactive decay a while before the manhattan project. They noticed things like induced radioactivity and nuclear fission, and found that these released energy as heat. From there, the idea of nuclear fission chain reactions emerged and countries then started research programs on the idea.