Once there was an old man. He had four sons. They had fallen into bad company. They mixed up with gamblers. The father advised them not to mix with such persons.
But the advice had no effect on them. One day, the old man bought a basketful of mangoes. He told the sons to eat the mangoes the next morning.
The old man kept one rotten mango among them. The next morning, the sons found that most of the mangoes were rotten. The father told them the cause. The sons learnt a lesson. They left the bad company.
Moral: A single rotten mango can spoil all others.