my lo does pretty well with sippy cups, esp if the smaller ones so they're easier to hold. For variety, I bought her two nuby flip up straw sippy cups with the silicone straws since so many posted how well their lo's liked straw cups. They have built in valves (which I didn't notice when I bought them), but I can't get them to work. I've tried putting water in both and can't suck anything through the straw myself- nothing comes through at all . Am I missing something? I've already destroyed the package, so I don't think I can take them back. I just feel like there's some kind of secret to it or something I'm not getting. I can't believe I'm being outsmarted by a cup...Anyone use these or know how to make them work?