With a serrated knife, cut a thick slice from the top and bottom of each grapefruit. One at a time, set the grapefruits on a cutting board, and with the serrated knife, cut downward toward the board, following the curve of the fruit, to completely remove the bitter white pith and the peel while leaving as much fruit behind as possible. Holding each grapefruit in your palm, cut downward between the segment dividers to remove the fruit sections.