- Covering something is a kind of protection
- A shell is a covering around the body of a shelled animal
- Hence, the shell of an animal is used to protect the animal

- The shell of an animal is used to protect the animal
- A shell is a part of a turtle
- Therefore, a shell is a part of a turtle for protecting the turtle

Therefore, a shell is a part of a turtle for protecting the turtle
Q: What is the most likely function of a turtle's hard shell?
Context:
- A shell is a part of a turtle 
- Covering something is a kind of protection 
- A shell is a covering around the body of a shelled animal