What makes an echo?
For grown-ups: the science
An echo happens when sound bounces off a hard surface and comes back to your ears.
An echo is reflected sound. A distinct echo is heard when the reflected sound arrives late enough for the brain to separate it from the original sound, usually because the reflecting surface is far away. Hard, smooth, large surfaces reflect sound well; soft materials absorb more sound.
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Transcript
What makes an echo sound like your voice came back?
You call, “Hello!” and your voice sends sound through the air. The sound moves away from you.
The sound reaches a hard, big wall, like a cliff or cave wall. It bounces back instead of going through.
The bouncing sound comes back to your ears a moment later. That returning sound is an echo!
So that's why an echo repeats your voice. Sound bounces off a hard, faraway surface and comes back!