Given a scientific passage and an answer, generate a question for the given answer.

Passage: The diaphragm is a sheet of muscle that spreads across the bottom of the rib cage. When the diaphragm contracts, the chest volume gets larger, and the lungs take in air. When the diaphragm relaxes, the chest volume gets smaller, and air is pushed out of the lungs. Answer: diaphragm
What is a sheet of muscle that spreads across the bottom of the rib cage?