How snakes came to Have Forked Tongues

Vinata was the mother of the Great Bird, Garuda, who is vehicle of Lord Vishnu.

One day Vinata had a fight with Kadru who was the mother of Nagas, the great snakes. Enraged by the insult caused to their mother, the Nagas captivated and enslaved Vinata. 

When Garuda came to the Nagas, asking them to release his mother Vinata; the Nagas conditioned, "We can release your mother from servitude if you fetch us Amrita (the nectar of immortality) from moon".

Garuda accepted the condition. He flew far and battled hard against Gods to obtain the pitcher of nectar. He flew back with the pitcher to the Nagas, asking them to release his mother.

The Nagas were pleased to receive the pitcher of nectar. They released Vinata. Garuda flew back with his mother.

The Nagas decided to wash themselves before they partook the nectar. They left the pitcher behind and went for bathing.

Lord Indra, the ruler of Kingdom of heaven had been following Gaurada after he took the pitcher of nectar from them. He had kept a close eye on the pitcher and when Nagas left for bathing, he quickly took the pitcher away. In haste some nectar was spilled on the darbha grass.

When the Nagas returned, much to their dismay they saw the pitcher of ambrosia missing. But they saw scattered drops of the nectar on the blades of darbha grass. They quickly began to lick over the scattered droplets of ambrosia. The Nagas didn't get much nectar. But their reckless moves over sharp grass blades slit their tongues, giving them forked appearance.