I have 11 month old twin girls. Both are normally really well behaved but recently, when i popped out of the room, one (it sounds harsh but it did look violent) attacked the other. The twin that can stand and walk was slamming the one who is still crawling on the head and pulling her hair. The twin being hit just sat there and cried, not even trying to push the other away. Is this normal behaviour? Is the one trying to assert her authority in some way? I've caught her three times now doing this and have always taken her away and said 'No' really firmly and tapped her on the hand. Twice she smiled and crawled back to re-attack. What can i do to deter her/let her know that this is wrong?