A few days after Lincoln's broken tooth incident, Maxwell was having a dispute with one of his siblings (hint: it wasn't Elsie) and he ended up getting pushed in the argument. He fell and hit his eye on the edge of the (carpeted!) stairs.
It immediately started swelling and the next day he had a pretty impressive shiner.
I guess he was just getting initiated into the black-eyed club.
We've got a nice little collection.