Theory:

There once lived a farmer with his wife. They lived in a village along with their baby. The farmer and his wife had an extreme love for their son. The farmer wished to have a pet in his home because he thought that it would be a good friend for his son. 
 
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Farmer's family
 
On a fine evening, the farmer brought home a small mongoose. The farmer and his wife thought that the mongoose is a friendly animal, and it is sweet as their baby. It would be the best friend for their son when they grow bigger.
 
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Baby mongoose 
 
Within five to six months, the mongoose had grown to its full size. While the boy was still a baby in the cradle, the mongoose became a loving pet in the farmers' house. Moreover, the adult mongoose looked so beautiful with shining black eyes and cute thick tail. While, the baby boy cried and slept repeatedly.
 
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Mongoose with bushy tail
 
The farmer's wife had to go out to buy groceries, and so she fed her baby and put him to sleep. Then she took a basket. And she asked her husband to keep the sleeping baby under careful observation. She didn't want to leave the baby alone with the mongoose.
 
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Farmer's wife

The farmer replied to his wife and said not to worry because the mongoose is a friendly animal. The farmer also said that it grew with the baby and it would not harm him. And in fact, the mongoose would protect him.

Then the wife went to the market. The farmer felt bored and went out to take a look at the fields and left the baby alone with the mongoose. On his way, he met some of his friends. It became quite late, and he didn't come home.

The farmer's wife finished her shopping from the grocery shop and returned home with a basket full of groceries. When she entered the house, she saw the mongoose sitting outside to welcome her as usual. The farmer's wife looked at the mongoose and noticed some blood smudged on its face and paws. And she screamed, "Blood!".
 
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Mongoose smudged with blood
 
She believed that the mongoose killed her baby. She screamed violently with uncontrollable emotion. She threw the basket full of groceries on the mongoose. After that, she ran inside to the baby's cradle.
 
She saw the baby sleeping, but there was a black snake was lying dead on the floor. The snake was covered with blood. At that moment, she understood what had happened. Then she ran out and looked at the mongoose. But the mongoose was already dead.
 
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Mongoose attacking the snake
 
She realised everything that she had done. She cried noisily and touched the dead mongoose because it had saved her baby's life by killing the snake.
  
The farmer's wife reacted impatiently without knowing the truth. After some time, she heard the baby crying. She wiped her tears and fed the baby.
 
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Farmer's wife in sorrow