My sermon for last Sunday, the Third Sunday of Lent, Year A:
One of the commonest metaphors in the Bible is the metaphor of hunger and thirst. Spiritually we have a need for God parallel to our physical need for food and water.
Jesus employed that metaphor when he said that man does not live on bread alone but on every word that comes from the mouth of God, and he uses the same kind of image here when he says that whoever drinks ordinary water will get thirsty again, whereas the water that he gives will turn into a spring, welling up to eternal life.