As little Robert was taking a walk, suddenly he shouted,"Ho, ho!"However, he was surprised to hear the same words after his voice.

Feeling playful, Robert said,"You must be a very silly person!"

"Silly person!"came back the words from somewhere on the hill.

Robert got very angry. He said more bad and mocking (讥讽的) words, but the words all came back to him in the same angry tone (音调). He decided to walk towards the hill to find out who was mocking him. 

However, when he got to the hill, he couldn't find anyone there. He was sad and confused, so he went back home and told his mother about the"bad boy".

His mother explained to him that when he spoke loudly near a hill, the sound waves (波) could bounce (反弹) back and make an echo. It was like throwing a ball against a wall and having it bounce back to you. The"bad boy"was just the echo of his own voice.

His mother went on,"If you speak kindly, you will hear a kind reply. "

Robert couldn't wait to run back out and try. This time, he spoke in a sweet, gentle tone, and the voice came back as sweet and gentle. Robert smiled.