When the soup stock has been strained and every particle of fat removed, return it to the kettle to boil. When it boils hard stir in carefully quarter of a cup of farina, do this slowly to prevent the farina from forming lumps. Stir into the soup bowl the yolk of one egg, add a teaspoon of cold water. Pour the soup into the bowl gradually and stir constantly until all has been poured into the bowl. Serve at once.