I don't know how many of you know that when we eat a heavy meal and we feel really miserable due to a stomach ache we say "Vatapi Jeernapi", so that the food which we eat gets digested. There is a beautiful story behind this...
Once during Sage Agastya's period there were two demons called Ilvala and Vatapi. These two demons were so jealous of the sages and the Brahmins. So they used to trouble a lot. Though Sage Agastya was a brahmin, he used to do severe penance and thus had the powers to digest even a stone. Vatapi had the power to take any form and Ilvala knew the Sanjeevini mantra. So Vatapi used to take the form of any food and Ilvala used to call the sages to have the food. When they eat the food, Ilvala used to call "Vatapi! come out." and Lo! Vatapi would tear the stomach of the sage and come out and the sage would be dead. Now, it was the turn of Sage Agastya. They did not know his powers and wanted to kill him.
As usual, Vatapi was transformed into goat meat and Ilvala invited Sage Agastya to have it. Sage Agastya knew what was going on, but without a hitch ate the meat. But as soon as he ate the meat, he said "Vatapi, Jeeranapi" three times and the food got digested. Ilvala started calling his brother, but where will he come from! He has already been digested. Sage Agastya didn't stop with this. He killed Ilvala also. Thus, he relieved all the Sages and Brahmins from the giants.