We buy those Sippy cups that have the valve built in. They are like $7.97 or something like that for 2 of them. I can't remember the exact brand but usually within 2 weeks they start getting black crap around the rubbery clear things that is in the lid. I've tried boiling them and I can't get it off. I try to make sure they get rinsed before going in the sink but not everyone else so sometimes they sit there for a few hours. Why are they doing that an how can I get it off? I love the cups but its expensive to keep buying more and I can't find just the lids.