The Evenflo Sensitive Response Clear Glass Bottle is designed with Micro Air Vents that let baby get an even flow of liquid and help keep air from entering baby's tummy. Each package includes six clear glass bottles, six natural silicone Stage 1 nipples, six cap rings, six sealing discs, and six nipple covers. The Slow Flow Stage 1 nipples are for 0-3 months. As your baby grows, you can replace the Stage 1 nipples with faster flow ones (check other auction for Stage 2 & Stage 3 nipples). Micro Air Vents allow air to return to the bottle, preventing nipple collapse and keeping air out of baby's tummy. Sensitive Response nipples are soft and flexible; and they adjust to baby's pressure while suckling for the proper flow of liquid.