Dr brown's options glass wide-neck bottle is the market's first convertible bottle that can be used with or without the vent system. it is now available in a glass version that some parents prefer . the unique internal green vent system continues to create 100% vacuum free feeding, which is clinically proven to help reduce wind related colic. this is what makes 9 out of 10 healthcare professionals recommend Dr. brown's. as baby becomes more developed the bottle can be used without the green vent system to work more like a conventional, teat-vented bottle. the options bottles are bpa free. the bottles are available in single or twin packs and silicone sleeves are also available to prevent heat transfer when using the glass bottles.