Hanuman who was on his way to Lanka in search of Mata Sita was stopped by Surasa, the sea monster. Surasa was the mother of Nagas. Devas had taken her help to test the strength of Hanuman.
Surasa stops Hanuman and states that He is her food for the day. Hanuman was in a hurry to reach Lanka and find Mata Sita. So He asks Surasa to leave him now and promises that He will come back and surrender to her after His mission is completed.
Surasa did not agree to Hanuman’s suggestion and kept blocking His way.
Hanuman finally agreed that she can eat Him. Hanuman reduced his size to that of a thumb and entered the mouth of Surasa and immediately flew out.
Hanuman thus fulfilled His promise.
Surasa was pleased with the intelligence of Hanuman and blessed him with all success.