Ephrem the Syrian on Genesis 1: “The Holy Spirit warmed the waters with a kind of vital warmth, even bringing them to a boil through intense head in order to make them fertile.  The action of a hen is similar.  It sits on its eggs, making them fertile through the warmth of incubation.  Here then, the Holy Spirit foreshadows the sacrament of holy baptism, prefiguring its arrival, so that the waters made fertile by the hovering of the same divine Spirit gave birth to the children of God.”