I have tried to put formula in it, and even if I cradle her she won't drink from it. I think she doesn't quite understand how to suck from it. Funnily enough she understands how to hold it from both hands and bring it to her mouth. I know liquid is coming out of it because I sucked some water out of it. I'm wondering if I should put some very watered down juice in it to see if that will peak her interest. I know she doesn't HAVE to use a sippy cup but right now in order to make sure she gets hydrated while eating solid food she drinks water from a bottle and I don't know if that makes it even harder for her to take the sippy cup.