system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can both fission and fusion release energy if they are opposites of each other?
gpt
Because in fission you're splitting large, complicated atoms that are held together loosely but release extra energy when split;

In fusion you're forcing together two small, light atoms which release energy when you join them.

If you try the reverse -- fusion with large atoms or fission with small atoms -- you don't get extra energy out, you lose a lot of the energy you put into it.

The middle point is iron:  it doesn't do fusion or fission in either direction very well, which is why stars end up with a lot of iron at their core when they die.