How do I get my son to drink from hard nipple sippy cups???? All he will drink milk from is soft nipple sippy cups.?? He can't even suck from a straw, but he can suck on those bottles with the Disney characters on top, those are hard, I also refill bug juice bottles with motts for tots, those are hard and he can Drink from that just fine? Anyone have any tips on how to get him to drink from regular sippy cups? :(