EXCELLENT BOTTLES.  THEY ARE DESIGNED TO HELP PREVENT GAS, AND THEY REALLY WORK.  MY GRANDDAUGHTER LOVES THEM.  IT WAS A BATTLE TO FEED HER UNTIL WE CHANGED TO THESE NEW BOTTLES.  NOW SHE FEEDS BETTER AND ALMOST NO GAS. GREAT RELIEVE FOR THE WHOLE FAMILY.  WONDERFUL BOTTLES, TRY THEM.  WE TRIED EVERY TYPE OUT THIER NOTHING WORKED BUT THE GERBER NUK.  THANKS GERBER.