Two AVENT Airflex 260ml bottles plus 2 slow teats use a unique teat skirt that has a one-way air valve that flexes to allow air into the bottle, eliminating the vacuum and making feeding easier for baby.The unique teat skirt has a one-way air valve that flexes to allow air into the bottle, replacing the milk at your baby's own suckling rhythm and eliminating the vacuum. This guarantees that the air is being released into the bottle, not into the milk or your baby's tummy so baby feeds effortless