lord vamana (lord vishnu) when asked bali for three feet land then bali laughed and said "brahamandeva (o brahmin) you might have asked many luxurious things from me as dakshina but you asked such a menial thing of land of a three feet quantity?" after which lord vamana grew so large in size as his one feet could cover whole earth and other whole heaven! that means, whole land property of bali was under control of lord vamana. as per bali's oath, there was no more land to donate and one feet of lord was remaining. bali asked lord about who is greater, the thing or who gives the thing? lord answer was the donar or giver of the thing is greater than the thing. then king bali asked the lord to keep his third feet on his (bali) head! pleased by this act of bali lord promised him the rule of 'patal' (the underearth) and promised to protect bali and his kingdom and slay whosoever disobeys bali in patal.