Sometimes, a person's body parts just have a life of their own. Literally. Variations include brain and blood cells, limbs, and the stomach.
Usually a comedy trope.
Anime & Manga
- In Parasyte, Shinichi's hand takes off a life of its own when it's infected by an alien parasite.
- In Osmosis Jones, all of the cells inside Frank's body are given a life of their own.