《Donkey and salt》说明：
In a villege there lived a donkey who carried heavy things on his back.
One day the donkey fell into the spring while carrying a bundle of salt.
The donkey shouted struggling.
"Help me,help me,please!"
The master rushed to the donkey and helped him up.
The donkey stood up and got surprsied.
"Why is it lighter?"
The salt melted in the water and it got lighter.
The donkey was happy.
Next day,the donkey had to carry things again. And he had an idea.
"Ah ha,yesterday it got lighter after i fell in to the water.Today I'll fall in the water again."
When the donkey was crossing the water he fell into it.
The donkey stood up with the help of his master and he got surprised again.
The things didn't get ligher but got heavier.
Today he was carrying cotton which getheavier when it gets wet with water.
The donkey regretted being sly.
