The Playtex pre-formed Drop-Ins liners contract as baby feeds, preventing a vacuum from forming in the bottle. The are clinically show to reduce colic, gas and spit up as no air mixes with the milk and baby doesn"t ingest air. Because the liner contracts, your baby can feed in a semi-upright feeding position, which is recommended by pediatricians to help reduce ear infections. Each liner is pre-sterilized for quick preparation and disposable for easy clean up. The new Playtex Drop-in liners expand to hold a full 10 oz of liquid for easier bottle preparation. For the first time, Mom can mix a full 8oz of formula right in the liner. Expandable drop-ins fit in all Playtex 8 oz nursers, are 100% leakproof, and are clinically shown to reduce colic. Available in 50 ct and 100 ct sizes.