The riddle
What Cannot Talk but Will Answer When Spoken To?
A reply with no speaker
The riddle describes a conversation that has only one participant. Wikipedia defines an echo as a reflection of sound that arrives at the listener with a delay after the direct sound. Nothing on the other side has chosen to answer, yet the voice comes back. The word echo comes from the Greek ēchō, from ēchos, meaning sound.
The delay measures distance
Wikipedia notes that the delay is directly proportional to the distance of the reflecting surface. Because the sound travels there and back, the distance is half of speed times delay. In air at 20 degrees Celsius sound covers about 343 meters per second, so a cliff 343 meters away returns an echo after 2 seconds. A closer wall brings it back sooner.
Echoes at sea
The National Oceanic and Atmospheric Administration explains that sonar, short for Sound Navigation and Ranging, sends a pulse of sound into the water, and when an object is in the path the sound bounces off and returns an echo to the instrument. By timing the gap between sending and receiving, the instrument works out the range of the object. A shout in a canyon and a ship's sonar use the same idea.
Where echoes live
Wikipedia lists typical places for an echo as the bottom of a well, a building, or the walls of an enclosed and empty room. Each of the three is a large surface facing the listener, and the delay in each case depends on how far away that surface stands.