system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 The heart pumps blood, the the liver filters stuff, the immune system protects us but what in our body produces the constant heat?
gpt
Metabolism.  When your body burns energy - when it converts those hydrocarbon chemicals (carbohydrates, proteins, and fats) and oxygen into carbon dioxide (and water and waste products), the process is not 100% efficient... and the lost energy is heat.  Just existing requires energy, and burning energy releases heat.