You shout 'Who goes there?' into a canyon and hear three responses. The second voice repeats exactly what you said, the third voice says 'I am the first voice.' Who is the third voice?
Hints
The second voice is a true echo; the third voice must be something else repeating that echo.
Answer
The third voice is the echo itself speaking again. The direct echo is the second voice, and the third voice, which claims to be the first voice, is just that same echo bouncing back yet again.