AVENT teats are made from soft pure medical grade silicone and shaped to mimic the breast in form and function. Breast fed babies will accept the AVENT teat more easily and will happily go back to the breast, so combining breast and bottle feeding is easier with AVENT. AVENT teats never collapse and have been clinically proven to reduce colic in newborn babies.IMPORTANT You must always use transparent teats on the AVENT Reusable and blue tinted teats on the AVENT Disposable, otherwise the bottles will leak. The transparent teat has a skirt at the base incorporating the anti-colic air valve. The blue teat has a flat base with no air valve, because the disposable bottle bag collapses as the baby feeds so there is no vacuum build up and no need for a valve.