I wanted to add this review for other parents attempting to feed their babies plastic free. I exclusively breastfeed my daughter, and due to an over supply, have to pump several times per day. Occasionally we give her a bottle of my pumped milk to make up for a missed feed or just so I can have a break. We bought these 4 oz glass mason jars from Walmart, and because of other reviews I saw here, bought these nipples to fit the jars. They fit perfectly, do not leak, and she prefers these nipples to the one on the life factory bottles. I highly recommend these nipples, especially for breast fed babies. A note on the jars: We freeze my milk directly in these jars, with the metal lid. Then thaw and replace the lid with the como tomo nipple. I hope this is helpful for other parents who want to avoid plastic.