Liners contract like the breast, preventing air from entering the system, so you can see baby gets just milk and no air. Bottle preparation is easy. Just place a pre-sterilized line into the bottle and fill, and then spend some more quality time with baby. And it doesn't hurt that cleanup is easy, too. See why the experts love it, too. All Playtex Baby products are BPA free. Do not boil any liquid, heat in microwave oven or sterilize inside the liner or nurser.