As told in the Bhagavata Purana, Kansa had learned that the child destined to end his reign was somewhere in Gokul. He could not rest. He summoned a demoness named Putana. She was cunning and gifted — she could take any form she wished.
Putana disguised herself as a beautiful, gentle woman. She wandered through the villages around Gokul. She carried poison in her body and gave it to young children under the guise of nursing them.
One day she arrived at Nanda Baba's house. Yashoda saw her — she was lovely and spoke sweetly. Yashoda had no reason to be suspicious. Putana picked up the infant Krishna and began to nurse him.
According to the Bhagavata Purana, the infant Krishna did not take in any poison. Instead, he drew away Putana's very life force. Putana writhed in pain. Her true monstrous form revealed itself. She crashed to the earth.
The sound of her fall shook all of Gokul. Yashoda came running. What she saw stopped her in her tracks — the infant Krishna was playing happily on the chest of the enormous fallen demoness. She rushed forward and took him in her arms.
The villagers crowded around. Everyone stared in disbelief at the huge form on the ground. But that tiny baby was completely unharmed. He was laughing.
The Bhagavata Purana tells us that this event made everyone in Gokul understand — this was no ordinary child. The village elders performed protective rites that day. And Yashoda never let Krishna out of her sight.