What? Wouldn't that be "Das Mädchen trinkt Milch"? This lesson is about plurals.
EDIT: There was previously an error in the translation. A reminder to everyone for when you see such a mistake is not to flood the comments page, but instead...
Second time this came up wrong. It's the girls drink milk, not the girl drinks milk.
We get it, stop with the discussion or read all the other posts before you resurrect this one again and again..
I fixed it a week ago. Could you please double-check that you really saw "Das Mädchen trinkt…"? Easy to confuse the two ;) A screenshot would be ideal.
I see two reports of "the girls are drinking milk". I think this might have to do with some caching problem, I'll check. My best guess would be that this issue should completely disappear soon.
As I mentioned ago, we (course contributors) fixed it a week ago, nothing we can do about it. The problem is known. It only seems to happen with a very small number of users. Sorry if you were one of them!