system
Given the following post, respond as if you're commenting on Reddit:
human
How does a turtle shell grow?

Is the shell like an exoskeleton? Is it even bone? Turtles aren't crabs and migrate from she'll to shell, therefore do they grow or does the title get big enough to push the shell and expand it?
gpt
Turtle shells are not exoskeletons. They are ribs covered in scales. Hence, they are made of bone and keratin, and grow just as your bones and fingernails do. The enlarged scales, called scutes, overlap each other and provide protection for the space between the ribs. Leatherbacks lack hardened shells, however, and their bones are just covered in skin.