You can fast from food  but not from water. In the Gospel Jesus uses water as a symbol to the Samaritan woman for slaking her thirst forever. He proposes to put a flowing fountain of water right inside her. Jesus is talking about the longing each of us has deep within for “the love poured forth from God in Jesus through the Holy Spirit.” That is the way Paul puts it in the second reading. This need of ours is much like our physical thirst except that it is more subtle. We use many other substitutes to fill it like food, work, looks, accomplishment, alcohol, other people, sexual satisfaction, and so on. They do not work. They leave us high and dry, croaking the famous line "Is that all there is?" The thirst for fullness is behind every move we make, even those, St. Thomas Aquinas reminds us, that are born of our misdirected and misguided longings.