Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Roundworms have a tough covering of cuticle on the surface of their body. It prevents their body from expanding. This allows the buildup of fluid pressure in their partial body cavity. The fluid pressure adds stiffness to the body. This provides a counterforce for the contraction of muscles, allowing roundworms to move easily over surfaces. Answer: tough cuticle covering
Which body part helps roundworm prevent their body from expanding?