If you own these bottles please pay close attention to the little vent holes/slits in the bottom disc. I was just washing my bottles and was horrified to realize that there was a black mold like substance in the slits. I wash them better than they recommend, I hand wash and boil all the pieces. So why is there mold forming in the slits? I'm getting rid of all these germ infested bottles and going to buy a better designed bottle. Oh...they don't prevent air bubbles any better than any other brand. Just don't waste your money on them.