The story is told that one day Christopher Columbus was sitting under an orange tree eating an orange, contemplating that if he sailed his ship to the horizon he would fall off the edge of the earth, when a butterfly came and landed on the top of his orange. As he watched the butterfly walk down the backside of the orange he realized that it did not fall off. Columbus reasoned that the same force of gravity that keeps the butterfly on the orange would keep his ship on the earth.