In plain words
Emperor Bali's daughter Ratnamālā: “How lovely this boy is! If He were born to me as a son, how good it would be!” she thought. All the scholars gathered there were spellbound on seeing His great radiance. Vāmana looked once with Prema at all who had gathered there. In every one of them a feeling of Bhakti towards Him arose.
Of all the thousands who had come to him to ask for gifts, Emperor Bali went first of all to Vāmana and asked, “Swami, what has brought You here?” “I have no need of wealth or grain, My dear! It is enough if you give three feet of land,” said Vāmana. “Why, even a dead body needs six feet! And You ask for only three feet! Is this all You ask for! If You want anything more, ask for it,” said Bali. “If you give three feet of ground, that is enough; it is as if you had given Me everything,” said Vāmana. Bali promised that he would surely give it. You must understand this not with a worldly eye but with a spiritual eye. Then Vāmana set down one foot: the whole earth was filled; He set down a second foot: the whole sky was filled. “Where will you give the third foot? If you give that too, I will go now,” He said. “Swami! Here, all that is left now is my head alone!” Saying this, Bali took off his crown and set it down, and Vāmana placed His foot on his head and pressed him down to the world of Pātāla. Bali was a great soul of Dharma, true; but pride ran high in him. That pride Vāmana pressed down. Then, as all the people grieved, “Swami, we will not get an emperor like this again,” Vāmana consoled them, saying, “Once a year, on this very day, Emperor Bali will come and see you; be ready.” That itself is today's Onam.
Today all the people of Kerala bathe, head and all, and cook ten kinds of dishes.