Teacher:In this task, you are given a paragraph, and your task is to construct a question from the given passage. Construct question in such a way that (i) it is well explained in the paragraph, (ii) the words may/may not be present in the paragraph, (iii) it should start with what/how/describe. Avoid creating a question that (i) is just relevant to one or two sentences, and (ii) not relevant to passage. 
Teacher: Now, understand the problem? Solve this instance: Something happens to produce a sound. The sound travels in waves. The sound waves hit an object. The object absorbs some of the sound waves. Some of the sound waves bounce off of the object. The reflected sound waves can be heard, though they have slightly different properties than the original sound. The sound continues to bounce off of things and produce echoes until it is totally absorbed or dissipated. 
Student:
How do echoes occur?