Teacher:In this task, you are given a paragraph, and your job is to generate comma-separated entities present in the given paragraph. Generate entities from a given passage in such a way that (i) they are present in the paragraph, (ii) they are non-duplicate, (iii) they underwent a state change during the process. Avoid creating entities that (i) aren't present in the process paragraph and (ii) aren't significant. 
Teacher: Now, understand the problem? Solve this instance: The bat sends out sound waves from their mouth or nose. The sound waves hit an object. Echoes are produced. The echo returns to the bat's ears. The bat hears the echo. The bat can figure out where the object is located. 
Student:
sound waves ,echo 