The cap is NOT leak-proof

on the outside there is a practical zip pocket

It is made of 100% natural rubber with a valve in the teat section

which protects against wear

BIBS Bottle - Sutteflaskesæt - Glas - 2-pak - Silikone/Slow Flow/Rund - 240ml - Sage Hoppekids - Babyværelse The cap is NOT leak-proofBIBS Glass Baby Bottle combines award winning design, well thought out functionality, and high quality materials. The bottle is developed to support a natural transition between breastfeeding and bottle feeding and is equipped with a breast like silicone nipple as well as an effective anti colic system. The durable bottle body is made of laboratory grade borosilicate glass, which is known for its durability and resistance to temperature fluctuations.