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

Passage: The stomach is a sac-like organ at the end of the esophagus. It has thick muscular walls that contract and relax to squeeze and mix food. This helps break the food into smaller pieces. It also helps mix the food with enzymes and other secretions in the stomach. For example, the stomach secretes the enzyme pepsin, which helps digest proteins. Answer: the stomach
What is a sac-like organ at the end of the esophagus?