In plain words
Teaching Dharmaja the Śānti Parva, Bhīṣma said he would tell a story about this. “A mother had an only son. Taking him with her, she was wandering through the forest. Being tired, she laid the boy on her lap under a tree and was resting herself too. Meanwhile there came a sound, ‘hiss, hiss’. By the time she opened her eyes to see what it was, the boy had cried out with a shriek. A snake had come out of an anthill, bitten the boy and gone back into the anthill. Seeing the boy cry, the mother fainted. The poison took effect, and the boy died. Holding the boy close, the mother sat wailing so that the whole forest could hear. Just then a hunter came there to rest. He could not bear the grief that mother was suffering. He asked what had caused it. ‘The snake from the anthill came, bit him and went away,’ the mother answered. The moment he heard this, he took up the crowbar he carried, broke open the anthill and tried to kill the snake. But she was a mother with faith in the fruit of karma, and so: ‘My son! It is not simply that a snake bit him; this is the karma my child himself made. That snake has no hatred for us, no anger. If a snake comes and bites for no reason, that is the karma my son made for himself. Just by your killing the snake, my son will not come back to life. So you need not kill that snake. Each one must undergo his own karma,’ she said. He was a hunter. He had killed many. He had no belief in the fruit of karma. So he grew angry. Had he had faith in karma, he would not kill anyone. ‘What is the matter with you, mother? You are taking pity on the snake that killed your son. However many people rest under this tree, the snake may kill them all!’ he said. Then that mother said, ‘My son! Rāma's foot touched so many stones, but did all of them take the form of a woman? No! Did all the stones turn into Ahalyā? Did all feet turn into Rāma's feet? Only when a foot like Rāma's foot and a stone like Ahalyā meet is the curse lifted.