Many have heard it, but nobody has ever seen it. It will not speak back until spoken to. What is it?

The correct answer to the riddle is an echo, which perfectly fits the description as something that is heard but never seen. An echo is a sound that has been reflected off a surface and returns to the listener's ear after the original sound has been produced. It does not "speak back" until a sound is made for it to reflect, making it a responsive, rather than originating, sound.

This fascinating phenomenon occurs due to the principles of sound wave reflection. When sound waves, which are vibrations traveling through a medium like air, hit a hard, solid surface such as a cliff, a large wall, or even the inside of a well or cave, they bounce back. For a distinct echo to be heard, there needs to be sufficient distance between the sound source and the reflecting surface, typically at least 17 meters (about 56 feet), so that the reflected sound arrives at least 0.1 seconds after the original sound, allowing the human ear and brain to perceive it as a separate event. Softer, porous surfaces, like curtains or grass, tend to absorb sound rather than reflect it, which is why you don't hear echoes everywhere.

Beyond being a curious auditory trick, echoes have significant practical applications. Many animals, including bats, dolphins, and some whales, utilize a sophisticated form of echolocation to navigate, hunt, and identify objects in their environment, especially in darkness or murky waters. They emit sounds and interpret the returning echoes to create a "sound map" of their surroundings. Humans have also harnessed this principle in technologies like sonar for underwater mapping and detection, and in medical imaging through ultrasound, which uses sound echoes to create pictures of internal body structures and even developing babies. The word "echo" itself has roots in Greek mythology, referring to a mountain nymph cursed to only repeat the last words spoken to her.