Here are five rephrasings (each 1–2 words shorter):
- An octopus can regrow a lost arm.
- Octopus can regrow its lost arm.
- An octopus can regrow lost arms.
- The octopus can regrow its arm.
- The octopus regrows its lost arm.
Simple explanation for a 6-year-old:
- Sometimes an octopus loses an arm.
- Its body grows a new arm to replace it.
- It takes time, but the new arm works like the old one.