'When a man is thus said to be thirsty, fire carries away what has been drunk by him. Therefore as they speak of a cow-leader (go-nâya), of a horse-leader (asva-nâya), of a man-leader (purusha-nâya), so they call fire udanyâ, thirst, i.e. water-leader. Thus (by water digested &c), my son, know this offshoot (the body) to be brought forth: this (body) could not be without a root (cause).

In plain words: The father explains thirst: when we are thirsty, it is because heat (fire) has used up the water we drank. So fire is called the 'leader of water.' The body also grows from water, and it must have a deeper root.