system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how rigor mortis works. Why does the human body get locked up in 6+ hours and then get flaccid again?
gpt
You’re muscles use a molecule called ATP to function, specifically it binds to muscle fibres and allow the connections in them to be broken, which allows the muscle to relax and the tension be released. 

When the body dies the muscles can no longer produce this ATP and so it runs out, leading to the muscles being unable to relax and thus rigor mortis.  

The muscles later become flaccid when the muscle fibres themselves begins decomposing and the connections are severed mechanically.