You will be given a definition of a task first, then some input of the task.
Given a scientific passage and an answer, generate a question for the given answer.

Passage: Did you ever hear an echo of your own voice? An echo occurs when sound waves bounce back from a surface that they can't pass through. The woman pictured here is trying to create an echo by shouting toward a large building. When the sound waves strike the wall of the building, most of them bounce back toward the woman, and she hears an echo of her voice. An echo is just one example of how waves interact with matter. Answer: echo
Output:
What is heard when sound waves bounce back from a surface that they can't pass through?