Those sippy cups can be a little tricky to clean. Especially, inside the top where the stopper is. I tried many things trying to get in the creases to clean out the gunk in milk. Finally after trial and error I figured out a great way to clean it. All you need is a toothpick. Yeap, a toothpick.
Check this out...
Here is a pic of before, you can't really see the gunk that's in there with a naked eye
Take the toothpick and get in the crease
Look what we didn't see