阅读英语短文,A lamb went out with some sheep one day.She found som

阅读英语短文,
A lamb went out with some sheep one day.She found some nice grass in the field,so she moved farther and farther away from the others.
She was enjoying herself so much that she did not know a wolf was coming towards her.The wolf quickly caught her and she shouted,"Please,please don't eat me.My stomach is full of grass.If you wait a while,I will taste much better."
The wolf thought that this was a good idea,so he sat down and waited.After a while,the lamb said,"If you allow mee to dance,the grass in my stomach will be digested faster."And the wolf agreed.
While the lamb was dancing,she had an idea.She said,"Please take the bell from around my neck.If you ring it as hard as you can,I'll be able to dance even faster."
The wolf rang it as hard as possible.The sheepherder heard the bell ring and quickly sent his dogs to look for the lamb.In the end,the barking dogs frightened the wolf away and saved the lamb.
How did the shepherd know the lamb was in danger?