Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Does not vent air

My baby tried these bottles and you could hear the air he was swallowing. He woke up that night much earlier than usual and had horrible gas. Dr. Brown's bottles are so much better and not much more $. Also, the extra nipples you buy with Dr. Brown's bottles come with 3, and the Avent come with 2 (doesn't make much sense if you buy a 3 pack Avent bottles).