I have twins who are 10 months old. They too like these cups, and ours leak just like yours. However, I have realized that there are two notches in the silicone 'nipple' or spout (whatever you want to call it). If those notches are not lined up correctly with the rim, the cup leaks. When you screw the lid to the cup, the notches become unaligned. You have to start the spout with the notches slightly unaligned and they will line up as you screw it on. Kind of a pain, but they don't leak that way.