Summary:

- Turtles have the unique ability to retract their heads and necks into their shells for protection. This is called "head withdrawal."
- The extent to which a turtle can tuck its head inside its shell varies by species. Some turtles can retract their heads almost completely, while others can only partially withdraw their heads.
- The mechanism that allows turtles to tuck their heads into their shells involves a complex system of muscles and bones that enables them to fold their necks in a zig-zag pattern. This helps turtles hide their vulnerable heads when threatened.